Rename the ledger vocabulary to chronicle across the repo #737

The upstream repo was renamed ledger → chronicle; microcosm's vocabulary should follow. One dedicated mechanical PR, landing right after #735 merges (adjudicated 2026-08-21: kept out of #735 so its semantic diff stays reviewable).

Blast radius (measured on the #735 branch): 142 files, ~4,960 case-insensitive occurrences. Scope:

• Modules: ledger_targets.py → chronicle_targets.py, ledger_artifact.py → chronicle_artifact.py (and uk_runtime/ledger_targets.py).
• Classes/functions: LedgerTargetReference, compile_ledger_target_references, load_ledger_consumer_artifact, ledger_target_registry_parity_report, … → chronicle spellings.
• CLI flags: --ledger-facts / --ledger-facts-sha256 / --ledger-manifest-sha256 → --chronicle-* (keep the old flags as hidden aliases for one release).
• Gate ids: ledger_compile_parity_*; resource filenames: uk/ledger_compile_parity_*_signed_differences.json; JSON keys inside committed resources (ledger_selector, ledger_* metadata stamps).
• Docs, comments, test names.

Compatibility constraints:

• The consumer-artifact schema string policyengine_ledger.consumer_artifact.v1 is emitted by chronicle's exporter — accept BOTH strings until a coordinated chronicle-side successor lands (file the chronicle issue alongside this one).
• Renaming JSON keys/gate ids/filenames moves content-addressed pins: re-pin the spec bundle SHA, CountrySpec fingerprints, and the _UK_GATE_BATTERY_* digests by recomputation.
• Follow the repo-rename checklist in the standards doc.

Acceptance: grep-zero for the old term outside compatibility shims and historical records; full suite green; all pins recomputed.