docs: the canonical-Rel plan and the corrected Windows baseline - #391

Two plan documents, landing after the work they describe.

docs/plans/fsroot-canonical-rel.md is issue #377's plan: rel canonical slash
form at every construction and decode site, abs OS-native, every non-test
Rel() consumer audited (two of them — walkDir and merkleRoot — were handing
native-form rel to io/fs APIs whose contract requires slash paths, so the fix
repaired them rather than risking them). Both rulings are recorded: existing
serialized documents are greenfield, and Abs()-class test expectations are
platform-correct rather than slash literals.

Its Outcome section records what actually happened: the implementation landed
inside PR #389 (the Apache-2.0 relicense) rather than as its own PR — it was
in-flight in the same working tree when the licensing sweep staged whole
directories, and the ruling was to merge with the entanglement disclosed in
the PR body rather than split commits or rewrite a shared branch. Issue #377
closed by that merge. Measured on #389's own windows test leg: 85 -> 57
failures, every fsroot/triad path assertion and TestRegistry_FilePaths
cleared, zero build failures. The attribution correction is recorded too: the
earlier ~50-of-85 estimate was wrong (the separator-direct share was 28), and
the surviving receipt/graph-resume round-trips have a second cause beyond
separator form — the first thing to read in the remaining-57 triage.

docs/plans/platform-test-matrix.md gains the corrected phase-2 baseline: with
the three formerly-[build failed] packages compiling on Windows (PR #388), the
honest count was 85, not 34 — the FAIL-line counting had silently excluded
three entire suites.

Documentation only; no code changes.
Issues: #377 (closed), #373