Add "extractionState": "manual" to every key by default

[adrianrl]

Summary

When managing strings outside the project (our keys are generated and live in a .xcstrings that isn't extracted from source), Xcode raised a warning on every key:

References to this key could not be found in source code. To re-use existing translations, copy and paste them into a different string using Paste Translations.

The fix is to set "extractionState" : "manual" on each key. POEditor doesn't add this on export, so poe now does it.

Changes

• download — marks every key as manual after normalizing placeholders. Done on the raw JSON tree (adds one field per entry, touches nothing else), so it's lossless: plurals and device variations survive untouched.
• StringCatalog serialization — defaults extractionState to manual when a key doesn't already carry one. This means distribute, filter and remove also guarantee manual on write, so .xcstrings files already committed to a project get fixed the next time any command rewrites them — no full re-download required. An explicit extractionState coming from the input is still respected.

Both paths are covered because our filter flow merges the working tree onto a baseline (the catalog already in the repo, which predates this behaviour); relying on download alone would leave the non-matching baseline keys warning forever.

Tests

• markingManualExtractionState marks every key and preserves plural variations; returns the input unchanged on invalid JSON.
• Serialization fixtures updated to reflect the new default.