Enable full compiler flag set (matches made/commons/regex)

[halotukozak]

Brings mcodec's project.scala up to the same //> using options set as made/commons/regex, including -Wsafe-init -Werror -Wunused:all.

Verified locally (JVM): full test suite passes clean with the new flags. Also confirmed -Wsafe-init does not hang here (unlike mrpc, where it's disabled for a documented reason) — 90s bound compile finished fine.

-Werror -Wunused:all surfaced 4 real warnings, fixed as follows:

• src/mcodec/CborInput.scala (skipChunks): the outer major param and each chunk's own major type (cm, from reader.readInitial()) were both unused — meaning a chunk's major type is never checked against the container's. Marked both explicitly unused (@unused/_) rather than silently adding validation logic, and left a TODO comment describing the gap: worth a look — malformed CBOR with mismatched chunk major types inside an indefinite-length container is currently accepted rather than rejected. Not fixed here since it's a behavior decision, not a compiler-flag cleanup.
• src/mcodec/Derivation.scala:11 (given self: MCodec[T] = deferred): flagged unused because -Wunused can't see that it's read via implicit search only after derivedRec's transparent inline body is expanded at call sites (needed for recursive/self-referential T). Suppressed with @nowarn rather than removed/renamed, to keep behavior identical — renaming it anonymous first (to test whether it was just the name) still triggered "unused local definition", confirming this isn't a naming issue.
• src/mcodec/Derivation.scala:119 (summonFrom presence-check pattern): same class of inline/macro false positive, suppressed with @nowarn on the expression.
• 3 unused test imports (ExtendedStdCodecsTest.scala, JavaHashSetIntTest.scala): removed, confirmed genuinely unused.
• test/mcodec/IoContractExtTest.scala: -Wsafe-init rejected floatCases/tsCases/binCases as not-yet-initialized when captured by test(...) closures declared earlier in the class body. Converted to lazy val — standard fix for this pattern, no behavior change (values are still computed once, just on first access instead of at construction).