ph7_vm_reset() previously cleared only the output blob, the script return
value and the HTTP response headers, so re-executing one compiled VM bled
globals, superglobals, function/class statics, runtime closures, output
buffers, the reference table and every allocated object into the next run --
and the -S server therefore recompiled the script on every request.

PH7_VmReset now restores the VM to its post-PH7_VmMakeReady state while
preserving the compiled program and all definitions:

- A watermark (nSuperBaseline, captured in PH7_VmMakeReady just before the
  superglobals are created) marks the per-exec/persistent boundary in the
  aMemObj object pool.
- Reset unlinks the whole reference table (before releasing objects, so by-ref
  array nodes never dangle), releases the per-exec object pool above the
  watermark (engine teardown only -- user __destruct is suppressed via a new
  bInReset flag to stay crash-safe, matching PH7's prior no-global-destructor
  behaviour), frees runtime closures and resets all function/method static
  sentinels in one pass over hFunction, frees class static typed-property
  slots, truncates the pool back to the watermark, then re-runs the real init
  helpers (VmEnterFrame, PH7_HashmapCreateSuper, and VmMountUserClassAttrs --
  the static/const-attr half split out of VmMountUserClass so methods are not
  re-installed).
- Definitions persist deliberately: include_once markers (aIncluded) are kept
  so runtime-included definitions survive without recompiling or redeclare
  errors, and user constants are not trimmed -- a re-run define() overwrites
  the value in place (PH7_VmRegisterConstant now frees the old user value on
  overwrite; case-insensitive define() aliases get their own value copy), so
  no leak and include_once+define keeps working across requests.

The -S dev server compiles each script once and reset-reuses it per request
via a small path-keyed VM cache with mtime invalidation (so edits are still
picked up; PHL_NO_REUSE=1 forces the legacy compile-per-request path).

Verified: no state bleed across requests (globals, statics, class statics,
closures, superglobals), definitions/constants persist across include_once,
flat memory over thousands of requests; make test + test-compat green under
phl and php 8.5.7, full corpus unchanged (24 known error-format failures).
Unblocks ESP32 B1.

Regression: tests/ph7/002-integration/server/vm_reuse_no_bleed.phpt