rt: exact-size heap bounds; n, the words new asked for, lives in bits 32..62 of p[-4]

the bnd recipe and paylen read n instead of the power-of-two block:
  bnd:    r6 = sub p 3; r6 = ld r6; r7 = xor p 0x2B7E1516; r6 = eq r6 r7;
          r7 = sub p 4; r7 = ld r7; r7 = shr r7 32; r7 = mul r7 q;
          r8 = mul i m; r8 = ltu r8 r7; r8 = xor r8 1; r0 = and r6 r8
  paylen: return p[-4] >> 32
two loads instead of three; osize, bchk, release, recycle and the memdebug
leak report all read n (rt.new(4) has length 4, rt.new(0) has 0).

writers: new_raw and new_k set n; stack.h line 87 and tinyvm.ts (BigInt) set
it on stack segments. release clears n when a block goes into quarantine
(len 0, indexing traps; under memdebug bchk reports a use after free); every
free-list path (fpush, mkblock, bdrop, recycle_leaf, balloc hold) rewrites
the word as k << 16, and an array that grows gets a fresh block. a request over
2^31-1-HDR words stops in new_raw (oom), and the compiler only takes the
new_k fast path for constant sizes that fit, so n is never truncated.

bit users checked (all mask or preserve bits 32..62; none compares or copies
the whole word): gc mark and sweep; release, free, recycle, recycle_leaf;
share and own; isobj, quar, checkptr, obj_base; bclass and the bfree buddy
merge; balloc, mkblock, fpush, funlink, heap_grow; frame_grow; stack_allocate
in src/core/stack.h and src/js/tinyvm.ts; micro.c and core.c (check word
only); the compiler's two p[-4] | F_INTERNAL writes (base.zc, builtins.zc).
32-bit ports: none (baremetal, wasm and micro use int64 Cell).

tests: tests/programs/exactsize.zc (n-1/n for non-power-of-two sizes, new(0),
packed bytes, recycle, grow, stack growth); 8-runtime: exact_size_bounds,
pointer_index_n_minus_1, pointer_index_n_traps, pointer_index_n_traps_memdebug,
new_zero_index_traps, new_too_big_traps, stale_index_after_free_traps,
stale_index_after_free_memdebug. expectations changed: leak report shows n
(tests/test.sh line 169, examples/learn/13-memdebug.out, docs/LEARN.md,
4-statements/memdebug_report_line.zc). programs adjusted to keep test.sh
expectations: quarantine.zc (unbounded), bounds_heap.zc (rt.new(11)).
spec 3.7.4 and 4.11 updated. fixpoint 402204 words (main 402197).

1 files changed, 2 insertions, 2 deletions

diff --git a/docs/SPEC.md b/docs/SPEC.md
index dd04cde..f422166 100644
--- a/docs/SPEC.md
+++ b/docs/SPEC.md
@@ -427,7 +427,7 @@ pn((int)(p + 1) - (int)p);    // 1: p + 1 moves one word
 
 3.7.3 `&x` is the address of a variable, field or element. it applies to a local (which then lives in memory and cannot be an o3 variable), a global, a field and an element of a non-packed array. an element of a packed array has no address (2.3.3).
 
-3.7.4 indexing a new object (`rt.new(int)[0]`) is error: index a new object only after binding it to a name. indexing is bounds checked at run time unless the function, block, statement or unit is unbounded or the code is o3 (4.15). a `T[?]` is checked against its length (3.13), a `T[n]` against `n`, and a pointer only when it points at the first word of a live heap block, against that block's payload (a buddy block rounds the payload up: `rt.new(4)` has 11 words). a pointer into the middle of a block, a frame array, a global or memory managed by hand is not checked. a constant index 0, and an index a loop test `i < k` with constant `k` (or `i < len(a)`) has already proved, are not checked again. a failed check prints `zy: index I out of range (length N)` and stops, as in 3.13.4.
+3.7.4 indexing a new object (`rt.new(int)[0]`) is error: index a new object only after binding it to a name. indexing is bounds checked at run time unless the function, block, statement or unit is unbounded or the code is o3 (4.15). a `T[?]` is checked against its length (3.13), a `T[n]` against `n`, and a pointer only when it points at the first word of a live heap block, against exactly the words new asked for (`rt.new(4)` has 4 words, though its block holds more; `rt.new(0)` has none). a freed block has none either: an index through a stale pointer to it is out of range (length 0), and under memdebug it is reported as a use after free. the runtime keeps that length in the block header, so a request over 2^31 - 6 words stops at the allocation with `zc: out of heap memory`. a pointer into the middle of a block, a frame array, a global or memory managed by hand is not checked. a constant index 0, and an index a loop test `i < k` with constant `k` (or `i < len(a)`) has already proved, are not checked again. a failed check prints `zy: index I out of range (length N)` and stops, as in 3.13.4.
 
 3.7.5 an index must be a number (typecheck on).
 
@@ -624,7 +624,7 @@ o3 int sum(fixed int[?] a) { int s = 0; for (int i = 0; i < len(a); i++) s += a[
 
 ### 4.11 memdebug
 
-`memdebug on;` turns on heap header checks and allocation-site tracking from this point in the file (top level) or from this point in the function (statement); the runtime enables the checks before `main` for the top-level form. `memdebug scrub;` additionally fills freed payloads with 57005; `memdebug off;` stops. with checks on, a use after free prints `zy: use after free from line N`, a double free prints `zy: double free from line N` and returns, a stray address prints `zy: bad heap pointer`. `rt.memreport()` prints a line `leak: line N W words` per live recorded object and returns the count.
+`memdebug on;` turns on heap header checks and allocation-site tracking from this point in the file (top level) or from this point in the function (statement); the runtime enables the checks before `main` for the top-level form. `memdebug scrub;` additionally fills freed payloads with 57005; `memdebug off;` stops. with checks on, a use after free prints `zy: use after free from line N`, a double free prints `zy: double free from line N` and returns, a stray address prints `zy: bad heap pointer`. `rt.memreport()` prints a line `leak: line N W words` per live recorded object (W is the n words new asked for, not the block's payload) and returns the count.
 
 ### 4.12 hints