fix(correctness): close Theme 10 — 5 data-integrity / serialisation findings

Final themed batch. 5 well-localised correctness fixes.

Serialisation precision:
- ESG-020: DatabaseGateway.JsonElementToParameterValue probes
  TryGetInt64 → TryGetDecimal → GetDouble, so a script's high-precision
  decimal SQL parameter survives the cached-write retry round-trip
  without silent precision loss. 3 new regression tests.

Template engine correctness:
- TE-018: DiffService gains ComputeConnectionsDiff over
  FlattenedConfiguration.Connections, mirroring the existing entity-diff
  shape and pairing with the Theme 1 TE-017 hash-coverage fix. A
  ConfigurationDiff record extension in Commons is flagged as a follow-up.
- TE-019: TemplateResolver.BuildInheritanceChain now walks via the
  int? ParentTemplateId directly — only null means "no parent". A real
  Id of 0 (the prior special-cased sentinel) now walks the chain like
  any other node, matching the TemplateEngine-013 CycleDetector fix.
  Regression of TE-013 closed.
- TE-020: All 5 Create* paths in TemplateService + SharedScriptService
  re-ordered to save-first → log-with-real-Id → save-audit (matching
  the InstanceService pattern). Create* audit rows no longer carry a
  literal "0" EntityId.

Doc deferral:
- Transport-012: Component-Transport.md §Audit Trail now spells out that
  the BundleImportId repository filter IS wired (in CentralUiRepository),
  but the Audit-Log-Viewer UI dropdown + summary-row hyperlink are a
  deferred CentralUI follow-up. CLI workaround documented
  (audit query --bundle-import-id).

11+ new regression tests (3 ESG, 4 DiffService, 3 TemplateResolver, 4
TemplateService, 1 SharedScriptService). Build clean; ESG 72/72,
TemplateEngine 324/324. README regenerated: 1 pending of 481 total.

Session-to-date: 135 of 136 originally-open Theme findings closed
across 10 themes in 10 commits.

code-reviews/ExternalSystemGateway/findings.md

@@ -8,7 +8,7 @@
 | Last reviewed | 2026-05-28 |
 | Reviewer | claude-agent |
 | Commit reviewed | `1eb6e97` |
-| Open findings | 1 |
+| Open findings | 0 |
 
 ## Summary
 
@@ -1118,9 +1118,11 @@ and produces a `"Timeout calling..."` (not `"Connection error to..."`) error.
 |--|--|
 | Severity | Medium |
 | Category | Correctness & logic bugs |
-| Status | Open |
+| Status | Resolved |
 | Location | `src/ScadaLink.ExternalSystemGateway/DatabaseGateway.cs:185-193` |
 
+**Resolution (2026-05-28):** `JsonElementToParameterValue` now probes `TryGetInt64` → `TryGetDecimal` → `GetDouble`, so a JSON number that fits in `decimal` materialises as a `decimal` (preserving the script's authored precision on cached-write retries) and only genuinely out-of-decimal-range values fall through to `double`. Regression test `JsonElementToParameterValue_DecimalShapedNumber_PreservesPrecisionViaDecimal` round-trips `1234567890.1234567890` through a `JsonElement` and asserts the result is a `decimal` carrying the original precision; companion tests guard the long-fast-path and the out-of-range-double fallback.
+
 **Description**
 
 `DatabaseGateway.JsonElementToParameterValue` materialises the buffered cached-write