SSDDigitizer: reuse TF1, hoist per-hit lookups, cleanup

[gavinsdavies]

Part of #303.

Fixes (all in Digitization/SSDDigitizer_module.cc)

• TF1 reuse: a TF1("func","gaus",...) was heap-allocated per hit in SimulateChargeSharing (and deleted at the end of the branch). Now created once in beginJob and reused via SetRange/SetParameters — removes per-hit allocation and gROOT name-registration churn. TF1::Integral is stateless given range+parameters, so results are identical. (Note: issue Broad-sweep code review: bugs, performance, and modernization findings (June 2026) #303 originally called this a leak; the original code did delete func — the cost was churn, not a leak.)
• ServiceHandle hoisting: art::ServiceHandle<ChannelMapService> was constructed inside per-hit branches (twice); now fetched once at the top of the function. Redundant inner re-declarations of echan/dchan removed (their fields are fully re-set before each use).
• adcRange(): the allowedADC table was a std::vector rebuilt on every call (per hit); now a static const std::array<float, 8>.

Cleanup (review feedback)

Removed using namespace emph; (everything lives inside namespace emph; DEFINE_ART_MODULE is fully qualified), dropped the commented-out legacy adcRange and debug std::cout blocks, and fixed tab/space indentation in the touched regions.

Verification

No numerical logic, thresholds, or outputs changed. No local build environment and no repo CI — a collaborator build before undrafting would be appreciated.

🤖 Assisted by Claude Code (claude-fable-5)

[FNALbuild]

✔️ CI build for EMPHATIC Succeeded on slf7 for maxopt -- details available through the CI dashboard