fix(types): emit working serde codecs for time::Date / time::Time (#25)

[lightsofapollo]

Fixes #25.

Problem

With DateStrategy::Time, format: date / format: time fields were emitted as

#[serde(with = "time::serde::iso8601")] // or ::option
pub proration_date: Option<time::Date>,

but time::serde::iso8601 only supports time::OffsetDateTime, so the generated code failed to compile (E0308, as reported).

Fix

The generated file now declares real codecs for these types via time::serde::format_description!, gated on actual usage (new TypeFeature::TimeDate / TimeTime, mirroring how base64_serde is gated):

time::serde::format_description!(time_date_format, Date, "[year]-[month]-[day]");
time::serde::format_description!(
    version = 2, time_time_format, Time,
    "[hour]:[minute]:[second][optional [.[subsecond]]]"
);

Fields then use with = "time_date_format" / "time_date_format::option" etc. The time format parses partial-time with or without fractional seconds; whole seconds serialize with a trailing .0 (valid RFC 3339, unavoidable since [optional] groups always format their contents).

Two latent bugs surfaced while verifying

• REQUIRED_DEPS for time was wrong even before this: features = ["serde"] doesn't enable time::serde::rfc3339 (needs formatting/parsing). The emitted line is now time = { version = "0.3", features = ["serde", "formatting", "parsing", "macros"] } (macros only pulled in when date/time codecs are used), and collect_dep_requirements unions feature lists for the same crate instead of dropping duplicates.
• Absent optional fields with a with codec failed to deserialize ("missing field") — a with attribute disables serde's implicit missing-field → None handling (Option fields require explicit default attribute if with attribute specified serde-rs/serde#2878). This also affected optional format: byte fields. Option-wrapped codec fields now also get #[serde(default)].

Testing

• New regression tests in tests/typed_scalars_test.rs covering required/optional date, time, date-time (still rfc3339, no helper emitted), and the merged dep line.
• Full cargo test suite passes.
• End-to-end: generated code from the issue's exact shape (optional format: date), compiled it in a scratch crate against time 0.3.53, and round-tripped present, absent, and fractional-second values successfully.
• scripts/spec-compile.sh smoke (anthropic/arcade/asana): all compile cleanly.

Side note: while testing I found that the README's documented [generator.types] TOML block is silently ignored — the parser reads top-level [types]. Filed as a separate beads issue (openapi-generator-cwd) since it's independent of this fix.

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