fix(date): brand-check this on Date.prototype getters (reflective dispatch)

[proggeramlug]

Summary

Date.prototype getters were installed as generic no-op thunks, so the reflective value path was broken:

Date.prototype.getDate.call(new Date(0))  →  [object Object]   (Node: 1)
Date.prototype.getDate.call({})           →  (no throw)        (Node: TypeError)
Date.prototype.getDate.call(Symbol())     →  (no throw)        (Node: TypeError)

Per spec every Date.prototype getter performs thisTimeValue(this), which throws a TypeError when this is not an Object with a [[DateValue]] slot.

Fix

Adds object/date_proto_thunks.rs (mirrors collection_proto_thunks / dataview_proto_thunks). Each getter thunk reads the IMPLICIT_THIS receiver, brand-checks it via is_date_value (throwing TypeError on mismatch), and otherwise dispatches to the same js_date_get_* helper the instance fast path uses — so reflective Date getter calls now both throw correctly and work correctly.

Covers getTime, valueOf, getFullYear/getMonth/getDate/getDay/getHours/getMinutes/getSeconds/getMilliseconds, all getUTC*, getTimezoneOffset, and legacy getYear. Installed after the OBJECT_PROTO_METHODS no-op block so the generic Object valueOf doesn't re-clobber Date.prototype.valueOf.

The instance fast path (d.getDate()) is lowered directly by codegen and is unchanged.

Results

test262 built-ins/Date: −36 (158 → 122 on top of the Date ToNumber fix #4393; no regressions). Verified Node-identical in both PERRY_NO_AUTO_OPTIMIZE=1 and default builds. cargo test -p perry-runtime --lib: 979 passed.

Independent of #4393 (disjoint files: this touches date_proto_thunks.rs/mod.rs/global_this.rs, #4393 touches date.rs). Setters / toX formatters keep the no-op path for now (follow-up).