fix(make): admit substitution before allocation

crates/bashkit/docs/threat-model.md

@@ -131,7 +131,7 @@ stdout capture limit remains downstream of pipelines and redirects.
 | Interactive terminal flooding (TM-DOS-119) | Unbounded type-ahead, undrained output, or a `vi` buffer grown by pastes | Caps on unread input (1 MiB), retained output (4 MiB), scrollback (1000 rows), transcript (1 MiB), line length, `vi` buffer (8 MiB) and undo history; `:w` goes through VFS limits | MITIGATED |
 | Input wait vs. timeout (TM-DOS-120) | Abusing the keyboard-wait exclusion to run work past the deadline | Only time blocked reading the terminal is excluded; CPU work and `sleep` still count | MITIGATED |
 | Concurrent pipeline amplification (TM-DOS-124) | An endless loop piped into `head`, a producer whose reader quit, or recursive pipelines `f() { f \| f; }` | 4 KiB pipes with backpressure; a writer whose reader is gone ends with 141 (SIGPIPE); streaming nesting counts toward `max_subshell_depth` and falls back to sequential stages past 4 levels; stages share the session budget, timeout and cancellation | MITIGATED |
-| make amplification (TM-DOS-126) | Exponential recursive variables, self-including makefiles, deep `$(MAKE)` recursion, huge graphs | Expansion depth 200 and 4 MiB per expansion; include depth 16; target, query and glob caps per run; `$(MAKE)` stops at level 4; recipes run under the session budget and timeout | MITIGATED |
+| make amplification (TM-DOS-126) | Exponential recursive variables, self-including makefiles, deep `$(MAKE)` recursion, huge graphs | Expansion depth 200 and 4 MiB per expansion; substitution checks its exact size before construction, with shared capacity leases retained by expanded values and work/deadline checkpoints while matching and copying; include depth 16; target, query and glob caps per run; `$(MAKE)` stops at level 4; recipes run under the session budget and timeout | MITIGATED |
 | Random generator amplification (TM-DOS-123) | `openssl rand` asked for gigabytes | Requests capped at 1 MiB; fixed-size `uuidgen`/`$SRANDOM` | MITIGATED |
 | Child-shell recursion (TM-DOS-125) | Script that re-runs itself via `sh` | Child shells count against function depth, max 8 nested | MITIGATED |
 | Pattern substitution (TM-DOS-127) | `${x//*a*/b}` over a long value | Globs run as a size-capped linear-time regex; extglob search capped at 10,000 attempts | MITIGATED |