fix(page-cluster): guard cross-block merges against cohesion collapse

[YusukeHirao]

Summary

@d-zero/page-cluster's Stage B cross-block merge could chain unrelated units together one merge at a time — each step individually passed its pairwise similarity check while the pooled group's actual common structure eroded toward nothing, producing large catch-all clusters. A degenerate L2 signature (too coarse to tell templates apart under a shared wrapper) made this worse by feeding indiscriminate merges into the same chain.

This PR:

• Adds a post-merge cohesion guard in front of every Stage B merge path: a proposed merge is rejected when the pooled group's quorum-core size, compared against each member's own original (pre-merge) core size, drops below a ratio threshold. The original core size is tracked via an anchor map that only ever grows (Math.max), so a long chain of individually-passing merges can't erode the reference itself.
• Adds L2 signature degeneracy detection: skips the L2 merge stage when too few distinct signature shapes exist among enough participants to be discriminating.
• Adds a post-hoc partition validation library (validateClusterPartition and its building blocks: detectMirrorAxis, normalizePathByMirrorAxis, normalizeHrefByMirrorAxis, computeClusterCohesion, findCrossClusterDuplicates, mergeValidatedClusters) that checks a finished clustering result against itself — cross-cluster duplicates that should have merged (optionally corroborated by a detected URL mirror axis, e.g. a language directory), and clusters whose members don't actually cohere with each other.
• Wires an opt-in onPartitionReport callback into resolvePageClusterKeys that, when provided, computes the report and applies its built-in safe-merge policy (byte-identical or mirror-axis-corroborated near-duplicates) to the output clusterKeys.
• Adds --validation-file to the CLI, new subpath exports for the added modules, and README coverage for the new checks.
• Adds a synthetic mirrored-template fixture reproducing the L2 signature collapse and the resulting catch-all merge, used by both the merge-cross-block-clusters and resolve-page-cluster-keys test suites.

Validated against several real crawl archives outside this repository; cluster quality held steady or improved across all of them with no crashes, including a 100k+ page corpus. No page-count, cluster-count, or site-identifying detail from that validation is recorded in this repo — all thresholds and behavior claims here are backed by the synthetic fixture only.

Test plan

• yarn lint — clean
• yarn test — 150 files / 1949 tests passing
• yarn build — all 29 packages build; page-cluster's new modules present in dist/
• Regression-checked against multiple real crawl archives (outside this repo) before and after the change
• /code-review medium — not yet run (requires manual invocation)

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