docs(m173): add spec — error-path coverage sweep

The M172 /write-unit-test audit classified every unhit line in the Zig tree:
2,431 across 317 files. 274 are errdefer rungs that only run when a later
allocation fails, 804 are failure response arms and their log lines, 94 are
error returns, and 1,243 are ordinary branches.

Five sections split by the mechanism that reaches each class, not by
directory, because every subtree contains all four classes. Allocation-failure
cleanup closes deterministically with std.testing.checkAllAllocationFailures;
ordinary branches get triaged into tested, deleted, or invariant-guarded
rather than padded with tests that assert only that a line ran.

Floors rise in the same commit as the tests that clear them, per the
raise-only discipline docs/architecture/testing.md already records.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>