Add the four ledger counters (held/pulled/returned/consumed) -- FABRIC-3.6.md task 2.4

FABRIC-3.5.md SXL.4's ledger: held == pulled - returned - consumed,
epsilon zero. Added sk_hermes_ledger() (an accessor, not a mutator)
plus four static counters in kernel_hermes.c.

Each counter has exactly one increment/decrement site: held/pulled
both move at sk_hermes_alloc()'s single success path, after every
refusal branch has already returned; held/returned both move at
sk_hermes_release()'s single success path. consumed is declared and
always reads 0 -- its one increment site doesn't exist yet, and won't
until task 2.5 gives decay something to record.

sk_hermes_release() now reads the Stadium cell's live header.heat
immediately before calling stadium_evict(), rather than assuming the
original pulled amount -- stadium_evict() zeroes the header as part of
freeing the cell and its own return value is a success code, not the
credited amount, so this is the only point the true remaining heat is
available. Today this always equals the original Q.SLOT pull; once
task 2.5's decay exists, this is what keeps returned correct without
touching this function again.

Self-test (kernel_main.c) extended: snapshots the ledger before
running so it checks its own deltas, verifies held/pulled grow by
exactly got_n * Q_SLOT on allocation with returned/consumed untouched,
then verifies held returns to its starting value and returned grows by
the same amount on release, and checks the audit invariant itself as a
bonus (task 2.6 formalizes this properly).

Three-arch boot clean: amd64/aarch64/riscv64 all reach [zuse@Hera] ok>,
zero UNKNOWN WORD, dict_hash unmoved. All three print PASS with
identical final ledger: held=0 pulled=65536 returned=65536 consumed=0.
No compiler warnings.

Authorized by Captain Bob ("keep going with rhe 6.5 document").

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

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