Normalize and validate host locales

[lukewilliamboswell]

Closes #439.

Root cause

The locale host functions forwarded OS strings without validating the public Locale invariant. They also replaced discovery failures with an invented en-US, making NotAvailable unreachable.

Fix

• normalize POSIX locale forms by trimming whitespace, stripping .encoding and @modifier suffixes, and replacing underscores with hyphens
• treat C and POSIX as non-language locales and omit them
• validate normalized values against the same pragmatic BCP 47 structure enforced by Locale.parse
• remove invalid values and case-insensitive duplicates while preserving preference order
• return NotAvailable when no valid preferred locale remains instead of inventing en-US
• document the normalization, rejection, and deduplication behavior
• add platform-independent tests covering representative Linux/POSIX, macOS, and Windows inputs

Verification

• cargo test --locked --lib (18 passed)
• ./scripts/test.py --operation validate with pinned nightly 2026-08-13-2fdd90e (207 Roc tests and all example checks passed)
• ./scripts/test.py --operation build --target x64musl --artifact-dir /tmp/basic-cli-439-artifacts
• native locale example with controlled environments:
  • LANG=C -> NotAvailable, []
  • LANG=en_US.UTF-8 -> en-US, [en-US]
  • LANG=malformed! -> NotAvailable, []

A strict Clippy run was also attempted; it is currently blocked by pre-existing warnings in generated ABI glue and unrelated TCP/file code.