fix(ergotree-ir): reject ErgoTree headers with version > 0 and no size bit

[Kshot3000]

Fixes #974.

The gap

The sigmastate-interpreter serializer runs CheckHeaderSizeBit (validation rule 1012) on the ErgoTree header byte during deserialization: for version > 0 the size bit must be set, otherwise a ValidationException is thrown — and because a tree without a size cannot be skipped, it escalates to a SerializerException and the whole parse fails. sigma-rust's ErgoTreeHeader::new accepted every version/flag combination, so ErgoTree::sigma_parse took the un-sized path for version 1/2/3 headers and interpreted the following bytes directly as constants + root. A trivial proof: take the canonical v0 serialization of a SigmaProp(true) tree and flip the header byte to 0x01 — it parsed as a valid v1 tree here, while the reference rejects it at the header byte.

The fix

ErgoTreeHeader::new now rejects version > 0 headers without the size bit with a new ErgoTreeHeaderError::MissingSizeBit(header_byte) variant. Putting the check in the header constructor means it covers both ErgoTreeHeader::sigma_parse and the full ErgoTree::sigma_parse path (which converts the error via the existing #[from] into SigmaParsingError), and it matches where the reference performs the check — on the header, before the body is read.

Behaviour matrix (all pinned by the new test):

• v0, no size bit: parses (canonical legacy form) — unchanged
• v0, size bit set: parses — unchanged (rule 1012 only fires for version != 0)
• v1..=v3, size bit set: parses (canonical sized form) — unchanged
• v1..=v3, no size bit: rejected (previously parsed under the wrong framing)
• v1 + constant segregation, no size bit (0x11): rejected as well

Tests

• Fail-first on pristine develop: the new test parse_header_version_without_size_bit_is_rejected failed at the first rejection assertion (header 0x01 accepted); the v0/v1-sized acceptance pins passed pre-fix, so in-range behaviour is pinned as unchanged, not just the new rejections.
• cargo test -p ergotree-ir --all-features: 265 passed (+2 doc/integration) — one new test over the 264 baseline.
• Downstream: cargo test -p ergo-lib --all-features --lib 121/121, cargo test -p ergotree-interpreter --all-features --lib 339/339 (the interpreter's own header construction in tests uses 0x0b — v3 with the size bit set — so it is unaffected).
• cargo clippy -p ergotree-ir --all-features --all-targets -- -D warnings clean; cargo fmt --check clean.

Deliberate non-changes

• The reserved header bits (5–7) are still ignored on parse, exactly as in the reference, which stores the raw header byte and only interprets version/segregation/size. This PR changes acceptance of version/size combinations only.
• ErgoTreeHeader::v0(..) / v1(..) constructors are untouched: they build valid combinations by construction (v1 always sets the size bit).

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