feat: definition-site anchors in breadcrumbs and cycle paths (ERR-6)

[lesnik512]

The accepted ERR-6 item from the 2026-07-05 3.0 UX research: breadcrumb steps and cycle paths carry the Factory creator's definition site as a trailing (module:line) anchor — a production traceback now names both ends of a failure and where each was declared.

• Capture — Factory.definition_site, lazy + memoized, error-path-only (zero declaration/import cost, verified by call-site grep): __code__.co_firstlineno free for functions, inspect.getsourcelines for classes, silent None for C callables/partials/pathological creators. Format module:line (maintainer-ruled), e.g. app.services:17.
• Breadcrumbs — ResolutionStep gains a defaulted location; chain lines render UserService (app.services:17). Alias/ContextProvider steps stay bare.
• Cycles — CircularDependencyError gains cycle_locations (public .cycle_path stays bare list[str]); both construction sites (validate DFS + RecursionError conversion) pass anchors.
• Never-raises invariant — anchor capture swallows everything (except Exception) with one reviewer-verified carve-out: a fresh RecursionError during capture propagates un-memoized so the runtime cycle guard's retry ladder re-converts with headroom — an instrumented probe confirmed anchors survive repeated stack exhaustion and no raw RecursionError escapes. Deterministic tests pin both branches.
• Docs — Symptom anchors on scope-not-initialized-error/scope-skipped-error/circular-dependency (the pages anchored tracebacks land on); scope-chain notes it carries no chain; architecture/resolution.md + validation.md promoted.
• Gates — just test-ci (100% line coverage, 337 passed), just lint-ci, just docs-build --strict, just check-planning all green.

Downstream note for the next release: exact-message assertions may break on the new trailing anchors; substring match= survives (2.24.0-style note to carry in the notes).

Design bundle (rationale home): planning/changes/2026-07-08.04-definition-sites.md

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