--emit-obj should record the class-ABI mode

Raised by [[feature-a-tobject-root-method-vmt-slots]], which made --compact-classes an ABI-splitting switch.

The hazard

--compact-classes sets the reserved root VMT slot count to 0; without it, 4. Every class's own virtuals therefore start at a different index. Two .o files built with different settings, linked together, disagree on what slot k means: a call through a base reference lands on the WRONG METHOD — no diagnostic, no crash at the call, just a plausible wrong result. Exactly the failure class CLAUDE.md's debugging note is about.

--platform=esp implies compact, so the mismatch is reachable without anyone typing the flag.

What to build

Record the mode in the emitted object (a note section / a symbol whose name encodes it — whichever costs less in elfwriter.inc), and refuse the link when two inputs disagree, naming both files and the flag. --threadsafe carries the same class of hazard and deserves the same treatment; do them together if the mechanism generalises.

Gate

make compiler/pascal26, tools/gate.sh quick, plus a test that links a compact object against a default one and asserts the REFUSAL (not a wrong answer).