Add sk_hermes_alloc(), the heat-coupled allocator -- FABRIC-3.6.md task 2.2, item 28

The piece FABRIC-3.5.md SXXXIII.6 calls "what remains genuinely hard,"
built and proven first per its own recommendation. Added
src/starkernel/vm/kernel_hermes.c (wired into Makefile.starkernel's
LOADER_EXTRA_SRCS -- this repo lists vm/*.c files explicitly, no glob)
and sk_hermes_alloc()'s declaration in kernel_hermes.h.

Checks stadium_reservoir_peek(vm_id) >= SK_HERMES_Q_SLOT before
touching the reservoir at all -- refusal this way needs no rollback,
since nothing was pulled -- with an explicit rollback path
(stadium_reservoir_push) kept defensively for the pull-then-short case,
though nothing in this single-core kernel is expected to reach it.

SK_HERMES_Q_SLOT = Q48_ONE / SK_HERMES_MSG_MAX (2048), deliberately
simpler than messaging.4th's own formula, which reserves a Q.1/3 floor
for COMMON-CH's own Stadium heat -- kernel-Hermes has no such object
(SXXXIII.4/SXXXIII.5's flat membership list carries no heat of its
own), so there is nothing left for that floor to protect.

Self-test in kernel_main.c, same diagnostic-only synthetic-VM pattern
as the existing Stadium quota grant self-test (lo=3, distinct from
that test's lo=1): reads back the actual granted reservoir rather than
assuming a number, derives expected_n from it, allocates to refusal,
and checks the refusal lands at exactly expected_n, the reservoir
doesn't move on the refused attempt (rollback proven, not assumed),
and the final reservoir is exactly reservoir0 minus got_n times
Q_SLOT.

Three-arch boot clean: amd64/aarch64/riscv64 all reach [zuse@Hera] ok>,
zero UNKNOWN WORD, dict_hash unmoved from task 2.1 (pure C, no FORTH
touched). All three print identical self-test arithmetic: reservoir0=
65536 Q_SLOT=2048 expected_n=32 got_n=32 reservoir_after=0. No compiler
warnings.

Noted, not fixed: Q_SLOT's divisor and SK_HERMES_MSG_MAX are the same
32, so reservoir and arena exhaustion land at exactly the same count by
construction -- this test can't distinguish which refusal reason
fired, only that refusal is correct and rolls back correctly.

Deliberately not evidence for stadium_conserved(): allocating alone
(no release yet, task 2.3) leaves pulled heat held off the Stadium
floor, so the two-term check would correctly read false right now if
run mid-hold. That's expected, not a bug -- Stage B (task 2.7) is
defined as "before and after the alloc/free cycle," not "continuously
during." This task's self-test checks reservoir arithmetic directly
instead.

Authorized by Captain Bob ("Yes continue").

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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