feat(adr0017): cold-image-as-ADR-0014-profile save path Verified

Adds src/cold-image.lisp implementing the cold/restart FUNCLIMG
profile codex specified in step (d): a strict subset of ADR 0014
with exactly three sections — CODE-TEXT, FN-TABLE, METADATA — and
none of the warm-tier machinery (HEAP, PACKAGES, TOPLEVEL,
CODE-MMAP, RELOC).  HEAP and TOPLEVEL absence is PROFILE
SEMANTICS, not an accidental omission; startup reconstruction
still runs, the image rides on top of it.

`funcl-cold --save-image PATH` now writes a FUNCLIMG:

  +0    magic        u64 "FUNCLIMG"
  +8    version      u32 = 1
  +12   arch         u32 = 1 (linux-x86-64)
  +16   body-size    u64
  +24   body-sha256  32 bytes (computed over body with this field
                              zeroed during hash, patched after)
  +56   section-count u32 = 3
  +60   section table (3 × 40 bytes)
  +180  CODE-TEXT   running binary bytes via /proc/self/exe
                    (mem-vaddr = +elf-load-vaddr+)
  +N    FN-TABLE    count + per entry: name-len + name + fn-ptr
                    drained from *ag-runtime-fn-addrs*
  +M    METADATA    32-byte build-id = sha256 of the running binary

For step (d) this commit lands SAVE ONLY — the matching --load-
image dispatch lands in the next commit with the integrity
checks (sha256, build-id match, FN-TABLE address-in-CODE-TEXT
validation).

Wiring:
  - Makefile: src/cold-image.lisp into COLD_PREFIX between
    cold-bootstrap.lisp and cold-loader.lisp.
  - src/main-cold.lisp: --save-image PATH dispatch, mirrors the
    existing --load-warm shape (null *argv2* → usage error).
  - tests/bootstrap.el funcl-cold-image-save-produces-well-formed-
    image: parses the saved image, asserts header magic + version
    + arch + body-size + section-count + section types are
    exactly {2,4,7} (no HEAP/CODE-MMAP/TOPLEVEL leakage) +
    CODE-TEXT mem-vaddr = +elf-load-vaddr+.

Cold binary grows from 1.04M → 1.06M (~20 KiB for the save
infrastructure + flag predicates).  Runtime fn-addrs count grew
from 1312 → 1335 from the new defuns.

Verified:
  - funcl-cold-image-save-produces-well-formed-image: new test passes
  - funcl-{cold,stage1}-self-host-fixed-point: byte-identical
  - funcl-cold-loads-warm-bundle: cross-region witness intact
  - funcl-warm-bundle-records-cross-region-call: contract test intact
  - full suite: only pre-existing
    funcl-stage1-supports-package-syntax-phase1 fails (unrelated)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>