feat(relinker): lower register-form EXTRQ and relocate following instructions for short sites
- **Lowering:** register-form EXTRQ is emitted out of line with SSE2 only. Two scratch XMM registers (distinct from both operands) are spilled; one gets the index (`src >> 8 & 63`), the other `(64 - length) & 63` (`length` masked to 6 bits, so 0 means 64). The destination is shifted right by the index, then left and right by the complement, which keeps the low `length` bits. INSERTQ register form is still reported as unsupported.
- **Short sites:** when a site is shorter than a `jmp rel32`, the converter extends it over the instructions that follow it until it's 5 bytes long:
- following EXTRQ/INSERTQ instructions (before any other instruction) are lowered into the same stub, via a new `MatchSequence`;
- other instructions are copied after the lowered body (`LowerOutOfLine`'s new `trailing` bytes), but only if they're sequential, have no RIP-relative operand and no branch target;
- anything else (a return, a branch, RIP-relative code, the segment ending) still fails the relink, as does a branch target inside the extended site.
- `docs/TechnicalDebt.md` now lists only the INSERTQ register form, MONITORX/MWAITX/CLZERO/RDPRU/MCOMMIT and SHA-NI.
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