perf: replace the per-file cache entries with a compact dir table

[benjaminbruun]

Every file cost a 28 B cache entry plus a malloc'd path string although only ten files can hold content at once; an 8 B hashed dir entry plus a fixed slot table cuts the 98-file profile's table from 6.7 KB of heap to 0.8 KB and makes the ten-buffer cap real. Validated on an nRF9151-DK: system-heap peak 13,524 -> 8,640 B, attach unchanged, and a pre-existing provisioned NVS is read as-is. The footprint report will show +160 B RAM (the static slot table); the 4.9 KB heap saving is invisible to it.

[github-actions]

Firmware footprint delta — nrf9151dk/nrf9151/ns

softsim_external_profile app image, from the ELF program headers (flash = load image incl. initialized data, RAM = static SRAM use incl. .bss/noinit):

Memory	base	PR	Δ
Flash	146.1 KiB	146.8 KiB	+708 B
RAM	72.3 KiB	72.5 KiB	+160 B

Section size changes (from the section headers)

     +14 B  .strtab              (65698 -> 65712)
    +128 B  .symtab              (82080 -> 82208)
    +160 B  bss                  (20722 -> 20882)
    +132 B  rodata               (34508 -> 34640)
    +580 B  text                 (111776 -> 112356)

Top symbol changes (10 largest, from the symbol tables)

    +288 B  ss_dir_table_from_blob
    +160 B  fs_slots
    +144 B  ss_fopen
    -130 B  generate_dir_table_from_blob
    +120 B  ss_slot_acquire
    -106 B  f_cache_find_buffer
    +100 B  port_provision
     +68 B  ss_dir_find
     +40 B  ss_file_size
     -34 B  f_cache_find_by_name

_{Soft report — informational only. Base 418e66207ab626f4a312fdcf6d5656eb97d5c712 vs PR head.}

[peterbornerup]

Overall very nice :D

Currently deleting files is half supported (at least the nvs key is removed). Creating files is not supported.

With this hashed approach updating the NVS DIR_ENTRY is gonna be harder as it can't be built from the context in memory. So the whole blob needs to be re-parsed, then updated and then re-written to the entry.

Small price to pay for such a big win memory wise :P