perf: dep_graph: serve edge lists in place from the retained file bytes [-0.64% incr-unchanged]

[xmakro]

Based on #44 (append-only save via wholesale carry of the record region). Split out of that PR to be measurable on its own; independent of the skip-edge-collection sibling PR #58.

Decoding used to copy every edge byte out of the file into a freshly allocated buffer, the largest allocation and copy of the load. The file stays mapped for the whole session anyway (the carry copies its record region forward), so serve the edge lists directly from the retained bytes: the on-disk varint encoding was already the in-memory representation, and each node's edge header now records a position in the file instead of a position in the copied buffer. Decoding a record shrinks to reading its fixed header and skipping over the edges. The fixed-size overread at the end of an edge list stays in bounds because the footer always follows the records. On Windows, where the mapping must not outlive the load because the save renames over the mapped file, the bytes are copied out once instead.

Local rustc-perf (primary crates, instructions:u), this change alone against the #44 branch:

scenario	delta
incr-unchanged	-0.64%
incr-patched	-0.43%
full / incr-full	-0.04% / -0.03% (control)

27 of 60 cells improved by at least 0.25%, 0 regressed.

Validation: 20 generations each of incr-unchanged, of alternating real edits, and of alternating edits under -Zincremental-verify-ich, all crossing the compaction boundary: every generation clean.