feat: JS/TS alias fixtures + unresolved-alias uncertainty note + docs (3/3)

[NehharShah]

What

Final PR of the issue-3 stack (stacked on #35, which is stacked on #34). Adds the acceptance fixtures, discloses dropped alias edges, and updates findings/README:

• Acceptance fixture test: a repo with Next.js-style @/… aliases and a pnpm-workspace @acme/core import produces exactly the edges the equivalent relative imports produce.
• deps.collect now takes an optional stats dict; imports that matched a tsconfig/jsconfig paths pattern but whose targets exist nowhere in the tree are counted as js_alias_unresolved (stored in kg.json as edge_stats), and findings.build turns a non-zero count into an uncertainty note surfaced by repokg audit — no silent coverage loss.
• JS/TS findings entry rewritten to reflect resolved alias coverage (kept at medium: extraction is still regex-based and extends chains / package exports maps are not modeled — the rationale now says exactly that).
• README: import-graph table row, Known limitations bullet, and roadmap checkbox updated.

Why

Resolves #3 (PR 3/3 of the stacked plan — closes the issue once the stack lands)

The acceptance criteria require alias/relative edge parity and unresolved aliases counted in an uncertainty note; both are implemented and tested here. Third-party bare imports that never matched a pattern are not counted — only pattern matches that failed to ground, which is the signal that coverage was actually lost.

Testing

• 3 new tests: alias-vs-relative edge parity on the combined Next.js + pnpm fixture, js_alias_unresolved counting (non-alias misses excluded), and the uncertainty note rendering from edge_stats
• Full suite: 80 tests, all green
• Dogfood: repokg scan . --no-github + repokg audit shows the new findings text; repokg clean round-trips

Checklist

• Branch is named issue-<N>/<short-description>
• Zero new runtime dependencies (stdlib only)
• Heuristics register findings with confidence + evidence (JS/TS entry updated; unresolved aliases become an uncertainty note)
• New artifacts are reversed by repokg clean (edge_stats lives inside kg.json, already cleaned)
• Tests added/updated and python -m unittest discover -s tests passes