restore STM poly shielding for run 2

[sdifalco]

Fix #1932: keep the STM poly shielding for run2

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☀️ The build tests passed at 0443c00.

Test	Result	Details
test with	✅	Command did not list any other PRs to include
merge	✅	Merged 0443c00 at da434b8
build (prof)	✅	Log file. Build time: 04 min 21 sec
ceSimReco	✅	Log file.
g4test_03MT	✅	Log file.
transportOnly	✅	Log file.
POT	✅	Log file.
g4study	✅	Log file.
cosmicSimReco	✅	Log file.
cosmicOffSpill	✅	Log file.
ceSteps	✅	Log file.
ceDigi	✅	Log file.
muDauSteps	✅	Log file.
ceMix	✅	Log file.
rootOverlaps	✅	Log file.
g4surfaceCheck	✅	Log file.
trigger	✅	Log file.
check_cmake	✅	Log file.
FIXME, TODO	✅	TODO (0) FIXME (0) in 0 files
clang-tidy	✅	0 errors 0 warnings
whitespace check	✅	no whitespace errors found

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[oksuzian]

PR Review Summary — #1934

Reviewed at head 0443c0097b023e128febba4e1be391ad85734b2b. First pass.

Decision

• 🟢 approve

This is the correction @YongyiBWu asked for on #1932, and it is right for the
reason he gave. Both findings below are about neighbouring files, not this diff.

Scope understood

• One line: geom_run2.txt goes back from STM_v10 (poly shielding removed) to
  STM_v09 (poly kept).
• #1932 bumped all five geometry configs to STM_v10; @YongyiBWu commented
  afterwards that "the change does not apply to geom_run2.txt. All the rest
  changes are good." This PR implements exactly that.

Findings

1. 🟡 [S2] geom_run1.txt is internally inconsistent by the same criterion —
   full downstream shielding with the poly removed

   • Evidence: @YongyiBWu's criterion on #1932 is that the poly may only be
     dropped where the blocks it is embedded in are gone — "the external
     shielding downstream blocks are removed in run1. The poly is embedded in
     the blocks so it cannot float in the air." Applying that to every config
     that references an STM version on main:

     config	STM	downstream shielding
     geom_run1.txt	v10	ExtShieldDownstream_v06 (full)
     geom_run1_a.txt	v10	reduced_ExtShieldDownstream_v06
     geom_run1_a_stickman.txt	v10	reduced_ExtShieldDownstream_v06
     geom_reduced_DSTS_shielding.txt	v10	reduced_ExtShieldDownstream_v06
     geom_run2.txt (this PR)	v09	ExtShieldDownstream_v06 (full)
     geom_2021_PhaseI_v03.txt	v09	ExtShieldDownstream_v06 (full)

     Every row lines up except geom_run1.txt, which pairs the full downstream
     shielding at line 52 with STM_v10 at line 63 — the same pairing this PR is
     fixing in geom_run2.txt.

   • Impact: with the blocks present and the poly not built, that config has an
     unfilled hole in the downstream shielding. It is a live file, not dead —
     EventDisplay/fcl/EventDisplayRun1.fcl and CRVReco/test/printChannelMap.fcl
     include it — though neither is production simulation, which is why this is
     S2 and not higher. It predates this PR and is not a regression from it.

   • Suggested fix: decide which of the two lines is the stale one. If the file is
     meant to describe the same shielding layout as geom_run2.txt, line 63 goes
     back to STM_v09 as here; if it is meant to be a run-1 layout, line 52 should
     be the reduced_ variant. Note @YongyiBWu did clear this file on #1932
     ("all the rest changes are good"), so there may be a reason I cannot see —
     worth a one-line confirmation either way.

2. ⚪ [S3] The reason for the split lives only in a PR comment

   • STM_v10.txt is #include STM_v09 plus one bool, with no comment saying why
     or when it applies, and the PR body says only "Fix #1932" — which is a merged
     PR, not an issue, so it does not close anything and the reader has to find
     @YongyiBWu's inline comment on it. A year from now the version numbers alone
     imply v10 supersedes v09, when in fact they are two live variants selected by
     the downstream shielding layout. One comment line in STM_v10.txt naming that
     condition would fix it, and it is the single place both files' readers land.

Verified, no action needed

• 🟢 The result is a clean revert: geom_run2.txt at this head is byte-identical
  (md5 bd1b6486a35b796254d38df7c607f273) to its state at a83d63f2, the last
  commit before #1932's d53f259f touched it. No other line moved.
• 🟢 The physics premise holds where I can check it. geom_run1_a.txt:52 includes
  reduced_ExtShieldDownstream_v06.txt while geom_run2.txt:52 includes the full
  ExtShieldDownstream_v06.txt, so run 2 does still build the blocks the poly sits
  in — which is the condition @YongyiBWu named for keeping it.
• 🟢 CI is green at this head: FNALbuild build 3295, all 20 rows ✅ including
  g4surfaceCheck, rootOverlaps, check_cmake, clang-tidy (0/0) and the
  whitespace check.

Validation check

• Build/tests run: CI green at 0443c009. Note the two overlap jobs do not
  exercise the changed file: surfaceCheck.fcl:38 points at geom_SurfaceCheck.txt,
  which includes geom_common_current.txt → geom_common.txt →
  geom_run1_a_stickman.txt. geom_run2.txt is in no CI chain, before or after
  this change, so the green tells you the build is fine and nothing about run 2's
  geometry. The revert-to-a-prior-state argument above is what covers that gap.
• Config contract check: pass — see the table in finding 1.
• Cross-repo consistency: n/a, single geometry text file.

Residual risk

• I took the embedding relationship — that the poly fills a hole in the downstream
  shielding blocks — from @YongyiBWu's description rather than reading
  ExtShieldDownstream_v06.txt against STM_v09.txt myself. Finding 1 rests on it.

Author follow-ups

1. Confirm or fix the geom_run1.txt line 52 / line 63 pairing (finding 1).
2. Optional: one comment line in STM_v10.txt recording that it applies only to
   configs using reduced_ExtShieldDownstream, so the v09/v10 split survives
   without the PR thread.