chore(codegen): split crates/perry-codegen/src/expr.rs (15k lines, one giant lower_expr match)

[proggeramlug]

Part of #1097.

Current state

crates/perry-codegen/src/expr.rs is 15,044 lines. The single function lower_expr spans roughly lines 1212 → 13483 (~12k lines) — one match arm per Expr::* variant, several of which are 500+ lines each. Helpers (lower_object_literal, lower_array_literal, lower_index_set_fast, lower_channel_reduction, try_lower_flat_const_index_get, the NaN-box inline helpers, the V8-export-call helpers, etc.) sit on either side.

The problem isn't conceptual — it's mechanical. Adding a new variant means scrolling past 10k+ lines, and ripgrep'ping for a helper named lower_object_literal returns the right hit instantly but you still land in a sea of unrelated arms.

Proposed split

Move the file into a directory module crates/perry-codegen/src/expr/ with:

• expr/mod.rs — keeps lower_expr as the entry point + the trivial arms (literals, LocalGet/LocalSet, Update, Undefined, Null, Bool, DateNow, etc.). Maybe ~1k–1.5k lines.
• expr/typeof_void.rs — Expr::TypeOf, Expr::Void
• expr/binary.rs — Expr::Binary arm (lines ~1732–2028)
• expr/unary.rs — Expr::Unary arm (~2028–2084)
• expr/compare.rs — Expr::Compare arm (~2084–2437)
• expr/object_literal.rs — Expr::Object arm + lower_object_literal helper (~14072–14260)
• expr/array_literal.rs — Expr::Array, Expr::ArraySpread arms + lower_array_literal (~14260–14439)
• expr/index.rs — Expr::IndexGet, lower_index_set_fast, try_lower_flat_const_index_get (~2507–2983, ~13518–13614, ~14439–14591)
• expr/property_get.rs — the cluster of Expr::PropertyGet arms (~2983–3200+, several distinct shapes)
• expr/v8_interop.rs — emit_v8_export_call, emit_v8_member_method_call, try_static_class_name, import_origin_suffix (lines 62–328)
• expr/nanbox_inline.rs — nanbox_pointer_inline, nanbox_bigint_inline, nanbox_string_inline, i32_bool_to_nanbox, unbox_to_i64, unbox_str_handle (~35–388, 13937–14046)
• expr/strings.rs — emit_string_literal_global, Expr::String, Expr::WtfString arms
• expr/channel.rs — try_match_channel_reduction, lower_channel_reduction, extract_array_of_object_shape (~14611–end)
• expr/i32_fast_path.rs — is_known_finite, can_lower_expr_as_i32, lower_expr_as_i32, try_flat_const_2d_int (~13483–13860)
• expr/url_helpers.rs — lower_url_string_getter
• expr/write_barrier.rs — emit_write_barrier, lower_stream_super_init

Each submodule re-exports its pub(crate) helpers; expr/mod.rs keeps pub(crate) fn lower_expr as the single dispatch entry point that delegates each variant to its module.

Acceptance

• Pure file moves: git diff -M -C90% shows ≥95% rename detection
• cargo test --release --workspace --exclude perry-ui-* green
• ./run_parity_tests.sh shows no regression vs. main
• Public surface (anything pub(crate) consumed outside expr.rs) preserved via re-exports