feat: CEP-22 - transport-agnostic oversized transfer framing engine

[harsh04044]

Part of #87
Builds the pure framing engine with no transport wiring - dead code until PR 3 (#87), but fully unit-tested.

What's in this PR:

New src/transport/oversized_transfer/ module:

• frame.rs - OversizedFrame enum (Start/Accept/Chunk/End/Abort) with serde tagging matching the TS wire format exactly
• constants.rs - all defaults aligned with TS (48KB chunk/threshold, 100MiB max, 5min transfer-timeout default - constant only, watchdog is wired in a later PR)
• errors.rs - 5-class error taxonomy (Abort/Policy/Digest/Reassembly/Sequence)
• codec.rs - SHA-256 digest, UTF-8 char-aware byte split (never breaks a multibyte codepoint), frame builder with correct progress slot layout
• receiver.rs - stateful OversizedTransferReceiver with admission control, out-of-order buffering, and triple validation at end (byte-len → SHA-256 → JSON-RPC parse)

Also adds validate_and_parse_oversized() to core/validation.rs - reassembled payloads can exceed the normal 1MB cap, bound to maxTransferBytes (100MiB).

One hardening over the TS reference: gap arithmetic uses i128 (vs the obvious i64) so a crafted u64::MAX progress value can't overflow or wrap - TS sidesteps this via JS floats.