feat: Migrate MPPersistence + DBMigrationController to Swift

[BrandonStalnaker]

What
Moves persistence schema logic, internal persistence models, and all persistence File I/O to Swift while retaining thin Objective-C controllers for SQLite operations.

Deletes the Objective-C implementations of:

MPSession
MPMessage
MPUpload
Their Swift replacements retain the existing Objective-C runtime names and selectors through @objc(MPSession), @objc(MPMessage), and @objc(MPUpload).

Persistence logic moved to Swift
MPPersistenceSchemaPRIVATE now owns:

Database versions and names
Paths and directory selection
Size limits
SQL statements and builders
Migration decisions
Legacy sidecar decisions
MPID table metadata
MPDatabaseMigrationLogicPRIVATE owns the SQL used to migrate records between database versions.

SQLite execution and row migration remain in Objective-C so raw SQLite pointers and Objective-C-only dependencies do not cross the module boundary.

Models moved fully to Swift
The Swift models preserve the existing behavior:

MPSession
Thread-safe properties
KVO notifications
Session timing and counter behavior
NSCopying
MPMessage
NSSecureCoding
JSON sanitization and truncation
NSCopying
MPUpload
Upload settings association
Serialization and copying
The Objective-C headers and implementations for these classes are removed.

File I/O and logger injection
Adds MPPersistenceFileSystemPRIVATE, which owns all persistence FileManager operations:

Database directory creation
Documents → Application Support migration
WAL, SHM, and journal sidecar migration
Legacy SessionNumber cleanup
Backup exclusion
Database deletion
Current and legacy path resolution
MPLog is injected from the existing MParticle logger, preserving current log-level and custom-logger behavior.

MPPersistenceController_PRIVATE and MPDatabaseMigrationController now delegate File I/O to this Swift implementation.

Behavior / API
Existing Objective-C runtime class names and selectors are preserved.
Published persistence signatures remain unchanged.
Database initialization and migration ordering is unchanged.
Failed directory migrations preserve the Documents database and continue using it.
Existing Application Support databases take precedence over legacy copies.
Orphan sidecars are removed rather than attached to another database.
No changelog changes; release automation handles them.
Validation
trunk check: clean for changed files.
iOS and tvOS simulator builds: succeeded.
Swift persistence File I/O tests: 9/9 passed.
Objective-C persistence integration tests: 62/62 passed.
Full Swift test suite: passed.
Full Objective-C suite passed

[cursor]

PR Summary

High Risk
Changes local database location/migration, event message encoding, and session/upload models that underpin all queued analytics and uploads—regressions could lose or mis-route persisted events.

Overview
Completes the Swift migration for core persistence data models by removing the Objective-C MPSession, MPMessage, and MPUpload wrappers. Swift types keep the same ObjC runtime names via @objc(MPSession), @objc(MPMessage), and @objc(MPUpload), with fuller implementations (thread-safe session state, NSSecureCoding on messages, upload settings on uploads).

Adds Swift-owned persistence metadata and file I/O: MPPersistenceSchemaPRIVATE (paths, schema SQL, size limits, migration decisions), MPDatabaseMigrationLogicPRIVATE (version-migration SQL), and MPPersistenceFileSystemPRIVATE (directory creation, Documents→Application Support migration, sidecars, backup exclusion). MPPersistenceController and MPDatabaseMigrationController still run SQLite but delegate path/schema/file work to Swift with injected MPLog.

Tests and mocks now import mParticle_Apple_SDK_Swift and use MPSessionPRIVATE where needed; MPUserDefaults.storedMpId() backs default session initialization.

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[github-actions]

🐦 Swift Migration Progress

Production implementation code at 3d9b5e44d422 compared with a16b1e84e8df.

Area	Goal	Progress	Base	This PR	Swift SLOC	Objective-C remaining	Change
Core SDK	Short term — in scope	████▏░░░░░	35.43%	41.04%	5,881	8,450	🚀 +5.61 pp
Core SDK	Long term — all Objective-C	██▉░░░░░░░	25.14%	29.13%	5,881	14,305	🚀 +3.99 pp
SDK kit infrastructure	Short term — in scope	░░░░░░░░░░	0.00%	0.00%	0	3,340	➖ 0.00 pp
SDK kit infrastructure	Long term — all Objective-C	░░░░░░░░░░	0.00%	0.00%	0	3,725	➖ 0.00 pp
Standalone kits	Short term — in scope	▏░░░░░░░░░	1.06%	1.06%	152	14,231	➖ 0.00 pp
Standalone kits	Long term — all Objective-C	▏░░░░░░░░░	1.06%	1.06%	152	14,231	➖ 0.00 pp

Objective-C retained by design: Core SDK 5,855 (-5) · SDK kit infrastructure 385 · Standalone kits 0.

This PR's code movement

Area	Swift lines added	Objective-C lines removed
Core SDK	1,024	1,012
SDK kit infrastructure	0	0
Standalone kits	0	0

How this is measured

• Current composition uses production source lines of code (SLOC) from cloc; comments and blank lines are excluded.
• Short term — in scope excludes the Objective-C the migration will not delete, so 100% is the end of this project: every in-scope implementation gone.
• Long term — all Objective-C keeps the full denominator. Reaching 100% there means the public API itself becomes Swift, which is a breaking change reserved for a future major release.
• The gap between the two rows is the retained public/kit contract, runtime-identity, and boundary-glue surface listed in Tools/swift-migration-retained-objc.txt.
• Retained wrappers keep their Objective-C interface but still shed logic to Swift. That thinning moves the long-term row and the retained figure, not the short-term row.
• Both revisions are measured with the manifest from the head revision, so a manifest edit does not by itself move the reported change. A retained file this pull request renamed or deleted still counts as retained at the base.
• Pull request movement uses physical additions/deletions from git diff base...head --numstat; it counts retained files too and is intentionally separate from SLOC totals.
• Core excludes SDK kit infrastructure and vendored libraries. Standalone kits include only files below Kits/**/Sources.
• Tests, examples, headers, build outputs, vendored libraries, and the MParticle/Sources Swift overlay are excluded.
• Objective-C++ (.mm) is included in the Objective-C figures and removed counts.

Generated with cloc 2.10. This report is informational and does not gate migration direction.

[github-actions]

📦 SDK Size Impact Report

Measures how much the SDK adds to an app's size (with-SDK minus without-SDK).

Metric	Target Branch	This PR	Change
App Bundle Impact	2.12 MB	2.14 MB	+24 KB
Executable Impact	848 bytes	848 bytes	+N/A
XCFramework Size	6.72 MB	6.68 MB	-44 KB

➡️ SDK size impact change is minimal.

Raw measurements

Target branch (workstation/swift-migration):

{"baseline_app_size_kb":84,"baseline_executable_size_bytes":75464,"with_sdk_app_size_kb":2256,"with_sdk_executable_size_bytes":76312,"sdk_impact_kb":2172,"sdk_executable_impact_bytes":848,"xcframework_size_kb":6880}

This PR:

{"baseline_app_size_kb":84,"baseline_executable_size_bytes":75464,"with_sdk_app_size_kb":2280,"with_sdk_executable_size_bytes":76312,"sdk_impact_kb":2196,"sdk_executable_impact_bytes":848,"xcframework_size_kb":6836}

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