TDD: PakRoundTripTests written first, confirmed a compile failure
against the not-yet-existing PakWriter/PakReader types.
PakWriter streams [header placeholder][64-byte-aligned blobs][TOC sorted
by key], then seeks back and finalizes the header once TocOffset/TocCount
are known (plan: "TOC last" so the writer doesn't need blob count
up front). PakReader mmaps the whole file once, loads the TOC into a
sorted array for O(log n) binary-search lookup, and verifies each blob's
CRC-32 LAZILY on first access (cached per-index so a corrupt blob logs
exactly once and is thereafter always treated as missing rather than
handing back garbage bytes). No locks anywhere — the mapped memory is
immutable for the reader's lifetime by construction.
CRC-32 implemented directly (standard IEEE 802.3 table-driven variant)
rather than adding a System.IO.Hashing package reference, keeping
AcDream.Content's dependency surface minimal per its existing
"no GL binaries" intent.
9 tests green: header round-trip, every-key-found, blob deep-equality
via the Task 3 comparator, missing-key returns false, corrupted-blob
(single flipped byte) is detected and treated as missing while leaving
sibling blobs unaffected, 64-byte blob alignment, and TOC binary search
cross-checked against a linear scan for both present and absent keys.
