fix(postgresql): filter PITR start WAL files

[weicao]

Summary

• select the PITR restore start boundary only from exact 24-hex PostgreSQL WAL segment files
• ignore unrelated regular files and incomplete/non-segment artifacts in pg_wal
• fail clearly when no valid local starting segment exists

Contract anchors

• docs/addon-continuous-backup-pitr-chain-integrity-guide.md, pit 4: continuous/PITR chains must not silently skip required segments
• docs/addon-api/09b-backup-extensions.md: restore inputs and artifact traversal must be validated before destructive handoff
• docs/addon-api/09c-restore-and-rebuild.md: PITR validation includes prepareData, continuous-chain selection, and the restored result

TDD evidence

RED commit 777265713 (Bash 3.2): 14 examples, 1 failure / 2 failed assertions. With valid local WAL001 plus unrelated README, prepareData selected README, skipped repository WAL002, returned success, and staged the restore.

GREEN commit cb7ad581c4f2884304dc121abbea37fc2cbef23b:

• focused PITR restore ShellSpec on Bash 3.2: 14 examples, 0 failures
• full PostgreSQL ShellSpec on Bash 5.3: 282 examples, 0 failures
• ShellCheck error severity, Bash syntax, and diff check: clean
• Helm lint: 1 chart, 0 failures
• Helm render: 41 documents, 1,013,743 bytes

Scope

Source/static product change only. Runtime package, environment, execution, cleanup, and formal repeated-full acceptance remain tester-owned and are not claimed here.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.00%. Comparing base (1fbbc0b) to head (cb7ad58).

Files with missing lines	Patch %	Lines
...ns/postgresql/scripts-ut-spec/pitr_restore_spec.sh	0.00%	14 Missing ⚠️

Additional details and impacted files

@@                             Coverage Diff                             @@
##           easton/pg-pitr-restore-destination-failfast   #3399   +/-   ##
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  Coverage                                         0.00%   0.00%           
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  Files                                              195     195           
  Lines                                            28346   28360   +14     
===========================================================================
- Misses                                           28346   28360   +14     

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