feat(synthesize): implement --locale option for 14 fake-rs locales

[jqnatividad]

Summary

• The --locale flag on qsv synthesize was previously a stub that rejected every value except EN. This PR makes it real: users can now request any of the 14 locales fake-rs 5.1.0 supports (en, fr_fr, de_de, it_it, pt_br, pt_pt, ja_jp, zh_cn, zh_tw, ar_sa, cy_gb, fa_ir, nl_nl, tr_tr) and get locale-appropriate fake values for dictionary-flagged columns (names, addresses, companies, etc.).
• Implementation uses a Locale enum plus a gen_faker_for_locale! macro that stamps out one near-identical generator function per locale — necessary because each fake-rs locale is a distinct Rust type, so dispatch has to happen via an enum match. The macro keeps the token→faker mapping in one place.
• Sparse locales (categories without per-locale data in fake-rs, e.g. lorem_* under fr_fr) silently fall back to EN trait defaults at runtime. This matches fake-rs's own Data trait semantics and means no per-category fallback logic is needed in qsv.
• --locale parsing is case-insensitive (FR_FR, fr_fr, Fr_Fr all work). Invalid values produce an Unsupported --locale '...'. Supported: ... error.

Implementation notes

• src/cmd/synthesize/faker_map.rs — new Locale enum, from_token parser, ALL slice for USAGE/errors, gen_faker_for_locale! macro, and 14 macro-stamped per-locale functions. Public content_type_to_value dispatches by locale.
• src/cmd/synthesize/generator.rs — threaded Locale through ColumnGenerator::build, ColumnGenerator::next, and build_faker_pool.
• src/cmd/synthesize/mod.rs — replaced the EN-only guard with Locale::from_token; updated USAGE help.
• USAGE help uses lowercase locale tokens to dodge docopt's "uppercase = positional arg" inference (this is a known qsv pitfall — uppercase tokens like EN, FR_FR in option descriptions break docopt's USAGE parser).

Test plan

• cargo build --locked --bin qsv -F all_features
• cargo test test_synthesize -F all_features — 12 passed (added 4 new: synthesize_rejects_invalid_locale, synthesize_accepts_fr_fr_locale, synthesize_locale_is_case_insensitive, synthesize_locale_changes_output; replaced the old synthesize_rejects_unsupported_locale)
• cargo test --bin qsv -F all_features synthesize:: — 15 passed (faker_map's per-locale tests now loop over all 14 locales × 40 vocab tokens for coverage + determinism)
• cargo +nightly fmt
• cargo clippy --bin qsv -F all_features -- -D warnings
• qsv --generate-help-md (refreshed docs/help/synthesize.md)
• Manual smoke test confirms --locale en / JA_JP / fr_fr produce English / Japanese / French names with seed 1.

🤖 Generated with Claude Code

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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🟢 Metrics 16 complexity

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Complexity	16

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[Copilot]

Pull request overview

Implements real multi-locale support for qsv synthesize --locale by adding a typed locale dispatch layer over fake-rs locales and threading the selected locale through synthesize’s generator pipeline.

Changes:

• Added a Locale enum + macro-generated per-locale faker mapping functions and locale-based dispatch for content_type_to_value.
• Threaded Locale through synthesize → ColumnGenerator build and value emission so faker-backed columns can be generated per locale.
• Updated CLI help/docs and expanded integration/unit tests to cover invalid locales, case-insensitive parsing, and locale-dependent output changes.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
tests/test_synthesize.rs	Adds/updates integration tests for locale parsing, case-insensitivity, and output differences across locales.
src/cmd/synthesize/mod.rs	Parses --locale into a typed enum and passes it into generator construction and row emission.
src/cmd/synthesize/generator.rs	Threads locale into faker pool generation and per-row faker/lorem generation paths.
src/cmd/synthesize/faker_map.rs	Introduces Locale, token parsing, macro-expanded per-locale faker mappings, and locale-aware dispatch.
docs/help/synthesize.md	Regenerates help text to document --locale behavior and supported locale tokens.

Comments suppressed due to low confidence (1)

tests/test_synthesize.rs:283

• Workdir::stdout does not assert status.success(). These comparisons can mask failures if the command exits non-zero but still prints something parseable on stdout. Consider asserting success on the command output before capturing stdout for equality/inequality checks.

    let mut upper = wrk.command("synthesize");
    upper
        .args(["-n", "5", "--seed", "1", "--locale", "FR_FR"])
        .arg("data.csv");
    let upper_out: String = wrk.stdout(&mut upper);

    let mut lower = wrk.command("synthesize");
    lower
        .args(["-n", "5", "--seed", "1", "--locale", "fr_fr"])
        .arg("data.csv");
    let lower_out: String = wrk.stdout(&mut lower);

    assert_eq!(
        upper_out, lower_out,
        "lowercase and uppercase locale tokens should be equivalent"
    );