Add ios_lint_localization_placeholder_changes action

[jkmassel]

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) — keys present in both whose signature differs, each { key:, old:, new:, old_signature:, new_signature: }. New/removed keys are ignored on purpose: copy that needs a fresh translation is expected to land under a new key.

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 (both int); %d↔%@ are not. %% is skipped; width/precision/length modifiers are ignored for type-classing.

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: abort_on_violations (default true) and allow_retry (interactive retry loop).

Worked examples (match the issue spec exactly — 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

• Android wrapper deferred to a follow-up, as the issue permits ("Android wrapper (or a follow-up issue)"). strings.xml parsing (Nokogiri, plurals, string-arrays, distinct escaping rules) is non-trivial and would roughly double this diff. The primitive is already platform-agnostic, so the follow-up is just 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 (open question in the issue; likely v2).
• No MIGRATION.md entry — purely additive.
• Heads-up: the diff is ~544 added lines vs Danger's 500-line guideline (275 of those are tests). Happy to trim the docs/heredocs if reviewers prefer a clean Danger run.

Test Plan

• bundle exec rubocop — no violations (full repo, 204 files).
• bundle exec rspec — 36 new examples pass (30 helper + 6 action); rest of the suite green. The one failing git_helper lfs test is environmental (git-lfs not installed locally) and unrelated to this change.
• Helper specs cover every worked example + edge cases (%%, nil/whitespace, non-ASCII, escapes, mixed positional/non-positional, width/precision/length modifiers, %d↔%i).
• Action specs write real .strings fixtures parsed by plutil: no-op, abort, abort_on_violations: false, add/remove ignored, retry-after-fix, missing file.

Related issues

• Implements the primitive + iOS wrapper from New action: localization placeholder-compatibility check (catch placeholder-breaking changes to existing keys) #736.
• 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.

Checklist before requesting a review

• Run bundle exec rubocop to test for code style violations and recommendations.
• Add Unit Tests (aka specs/*_spec.rb) if applicable.
• Run bundle exec rspec to run the whole test suite and ensure all your tests pass.
• Make sure you added an entry in the CHANGELOG.md file to describe your changes under the appropriate existing ### subsection of the existing ## Trunk section.
• If applicable, add an entry in the MIGRATION.md file — N/A, purely additive.

[dangermattic]

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