Add eq_ignore_ascii_case for char and String

[gfx]

Summary

• Add char::eq_ignore_ascii_case(&self, other: &char) -> bool — delegates to to_ascii_lowercase comparison, matching Rust's implementation
• Add String::eq_ignore_ascii_case(&self, other: String) -> bool — byte-level comparison using per-byte to_ascii_lowercase, matching Rust's str::eq_ignore_ascii_case
• Make char::to_ascii_lowercase and char::to_ascii_uppercase branchless, matching Rust's u8 implementation pattern: c | (is_upper as u32 * ('A' ^ 'a'))

Test plan

• primitive_test.wado: 5 test cases for char::eq_ignore_ascii_case (same case, different case, non-alpha, different letters, non-alpha with same bit pattern)
• string_test.wado: 6 test cases for String::eq_ignore_ascii_case (same case, different case, non-alpha, different lengths, empty, non-ASCII)
• All existing char and string tests pass across all optimization levels

https://claude.ai/code/session_01YTKHy6Dzb84sH6hyxVEDXz

[github-actions]

Working tree is dirty

Integrity checks produced the following changes:

 docs/cheatsheet-stdlib-core.md                     |   2 +
 docs/stdlib-core.md                                |  12 ++
 .../fixtures.golden/iterator_trait_map.wir.wado    | 106 +++++++++--------
 .../fixtures.golden/string-truncate-panic.wir.wado |   4 +-
 .../tests/format.fixtures.golden/all.lower.wado    | 130 +++++++++++++--------
 .../tests/format.fixtures.golden/mess.lower.wado   | 130 +++++++++++++--------
 .../format.fixtures.golden/ops.all.lower.wado      | 130 +++++++++++++--------
 .../format.fixtures.golden/ops.mess.lower.wado     | 130 +++++++++++++--------
 8 files changed, 394 insertions(+), 250 deletions(-)

Please run mise run on-task-done locally and commit the changes.