Add ios_lint_localization_placeholder_changes action

[jkmassel]

Depends on #741. The .strings parsing and duplicate-key detection this action builds on were extracted there, so this PR is now just the primitive + the action. Review/merge #741 first; GitHub will retarget this PR to trunk once it lands.

What does it do?

Adds a placeholder-compatibility guardrail for localization source strings, as requested in #736.

Existing translations are keyed by a string's key, not its English text. If an existing key's source value changes its format placeholders — count, position, or argument type — every translation filed under that key silently breaks (or crashes) at the new call site. This is a different axis from ios_lint_localizations, which compares each translation against the base language at a single point in time; this compares base ↔ base across versions (temporal).

1. Reusable primitive — Fastlane::Helper::StringPlaceholdersHelper

Pure Ruby, no file I/O, platform-agnostic (the %@ / %1$d placeholder syntax is shared by iOS and Android):

• placeholder_signature(value) — canonical signature of a value's placeholders ('' if none), e.g. "1:int,2:object".
• placeholders_compatible?(a, b) — signature(a) == signature(b).
• incompatible_placeholder_changes(old_hash, new_hash) — the keys present in both whose signature differs. New/removed keys are ignored on purpose: copy that needs a fresh translation is expected to land under a new key.
• mixed_operators?(value) — whether a value invalidly mixes positional (%1$@) and non-positional (%@) specifiers.

The signature is invariant to benign copy edits and to reordering equivalent positional args, but sensitive to count/position/type changes. %d↔%i are compatible; %% is skipped; the ,/' grouping flags are recognized; * dynamic width/precision and the write-back %n are deliberately left unrecognized. A single literal % followed by whitespace is treated as a literal percent, not a specifier, so ordinary copy like "50% off" doesn't fabricate a phantom placeholder.

2. iOS action — ios_lint_localization_placeholder_changes

Takes old_file / new_file (the previous and newly-generated base .strings), parses them via the existing L10nHelper.read_strings_file_as_hash (plutil), runs the primitive, and reports. Mirrors ios_lint_localizations's abort_on_violations (default true).

It fails closed on any input where the comparison would be unreliable:

• check_duplicate_keys (default true) — aborts when either file defines a key more than once (plutil silently keeps only the last value, which could otherwise hide a real placeholder change). Delegates to the shared StringsFileValidationHelper.scan_for_duplicate_keys from Improve .strings handling: unquoted strings, inline comments, and merge prefixing #741.
• Always aborts (independently of abort_on_violations) when a value mixes positional and non-positional placeholders — that's invalid and leaves the signature undefined.

Worked examples (match the issue spec — verified by tests)

Value	Signature
Just text	(empty)
Hello %@	1:object
%1$d items in %2$@	1:int,2:object
%2$@ told by %1$@	1:object,2:object
100%% sure about %@	1:object
%d likes	1:int

Notes / scope

• Builds on Improve .strings handling: unquoted strings, inline comments, and merge prefixing #741, which carries the .strings parsing (unquoted keys/values, inter-token comments) and the scan_for_duplicate_keys helper this action calls.
• Android wrapper deferred to a follow-up, as the issue permits — the primitive is already platform-agnostic, so the follow-up is a thin android_* parse-and-call wrapper.
• Reuses L10nHelper.read_strings_file_as_hash rather than adding a second .strings parser — the action is therefore macOS-only by design, like ios_lint_localizations.
• .stringsdict / ICU plurals are out of scope (likely v2).
• No MIGRATION.md entry — purely additive.

Test Plan

• bundle exec rubocop — no violations.
• bundle exec rspec — all green. Helper specs cover every worked example + edge cases (%%, a literal % in copy, nil/whitespace, non-ASCII, escapes, width/precision/length modifiers, %d↔%i, the ,/' grouping flags, * dynamic width/precision, %n, %.f, and the invalid positional/non-positional mix). Action specs write real .strings fixtures parsed by plutil: no-op, abort, abort_on_violations: false, add/remove ignored, duplicate-key abort, fail-closed on an un-tokenizable file, mixed-operator abort, and missing file.

Related issues

• Implements the primitive + iOS wrapper from New action: localization placeholder-compatibility check (catch placeholder-breaking changes to existing keys) #736.
• Foundation (.strings parsing + scan_for_duplicate_keys) extracted to Improve .strings handling: unquoted strings, inline comments, and merge prefixing #741.
• Reference implementation: Add continuous translations (Faster Releases, Phase 2) WordPress-iOS#25675 (fastlane/helpers/string_placeholders.rb + the validate_string_placeholders lane). WPiOS will adopt the toolkit version and delete the project-side copy once this lands.

[dangermattic]

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