Fix N-RUNS/START-REP hardcoded constants; N-REPS increase blocked on reservoir cost, not a bug (FABRIC-3.md §XXIII)

Two real bugs found and fixed while attempting to raise N-REPS from
3 (both harmless only by coincidence at N-REPS=3, since 3 happened
to equal the hardcoded/literal values):

- N-RUNS was `27 CONSTANT`, not derived -- now `N-ID N-REPS *
  CONSTANT N-RUNS`.
- START-REP's run_id decode used a literal `3 *` where it meant
  `N-REPS *` (confirmed against EXEC-STD79-DOE, the serialized
  baseline, which correctly uses N-REPS for the same decode).

Re-verified at N-REPS=3 (amd64): byte-identical to the already-
verified baseline -- 193s, 0 faults, 99/99 tokens every identity,
K conserved on all 656 rows.

Raising N-REPS to 6 was then tested and found to cause real, silent
data loss at full 8-identity scale (rajames 0/198, 00 66/198, 01/02
99/198, 03-06 fully complete) -- same failure class as §XXI defect 2,
just past the budget again since doubling N-REPS roughly doubles
total MSG-SEND volume (24->48).

Measured the reservoir's replenishment directly rather than assume
from source: 40 sends drained it from 21735 to 1; 300s of pure idle
time (zero further sends) brought it back to 2049 -- real, but only
~6.8 units/s, meaning a full refill would take on the order of 53
minutes against a campaign's few-hundred-second runtime. Raising
N-REPS further needs a cheaper per-send cost or an explicit top-up,
not reliance on ambient decay.

Reverted N-REPS to 3 (fixes kept, they're correctness fixes
independent of the value) rather than commit a silently-lossy result.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EXieurDfDSsDFdnSyusuWo

experiments/std79-doe/std79-doe.fth

@@ -9,7 +9,7 @@
 
 9 CONSTANT N-ID
 3 CONSTANT N-REPS
-27 CONSTANT N-RUNS
+N-ID N-REPS * CONSTANT N-RUNS
 24 CONSTANT N-TESTS
 
 : ID-NAME ( idx -- c-addr u )
@@ -258,7 +258,7 @@ CREATE REV-MATRIX N-RUNS CELLS ALLOT
 : REV@ ( idx -- val ) CELLS REV-MATRIX + @ ;
 : BUILD-REV-MATRIX ( -- ) N-RUNS 0 DO I  I MATRIX@  REV! LOOP ;
 
-CREATE REP-IDX N-ID CELLS ALLOT   ( each identity's current/next rep, 0..2; N-REPS = done )
+CREATE REP-IDX N-ID CELLS ALLOT   ( each identity's current/next rep, 0..N-REPS-1; N-REPS = done )
 : REP-IDX! ( val idx -- ) CELLS REP-IDX + ! ;
 : REP-IDX@ ( idx -- val ) CELLS REP-IDX + @ ;
 
@@ -342,7 +342,7 @@ VARIABLE SR-IDX  VARIABLE SR-REP  VARIABLE SR-RUNID
 : START-REP ( idx -- )
   SR-IDX !
   SR-IDX @ REP-IDX@ SR-REP !
-  SR-IDX @ 3 * SR-REP @ + REV@ SR-RUNID !
+  SR-IDX @ N-REPS * SR-REP @ + REV@ SR-RUNID !
   ." DOE-RUN," SR-RUNID @ . ." ," SR-IDX @ . ." ,"
   SR-IDX @ ID-LABEL TYPE ." ," SR-REP @ . CR ;