fix(schema): preserve quoted column name parts

[gudcks0305]

Column(String) strips quoting from the complete qualified name before splitting it, corrupting individually quoted components:

new Column("t1.`id`").toString();
// Before: "t1"."`id"
// After:  t1.`id`

Split qualified names at dots outside quoted components and preserve each component's representation. Keep already-separated column components intact while retaining the existing BigQuery whole-quoted namespace behavior.

Regression coverage includes mixed quoting, quoted dots and escaped delimiters, constructor/setter paths and AST getters, splitting-off controls, PostgreSQL identifiers, and BigQuery namespaces.

Validation on macOS with JDK 17:

• Regression tests fail on the unchanged upstream base, then pass with the fix.

• Focused schema/PostgreSQL/BigQuery tests: 162 tests, zero failures/errors, one existing skip.

• ./gradlew check --no-daemon --no-build-cache: 9,328 tests, zero failures/errors, 25 existing skips; grammar ambiguity, formatting, static analysis, and coverage checks pass.

• mvn --batch-mode --no-transfer-progress -Dmaven.javadoc.disableNoFonts=false verify: 9,310 tests, zero failures/errors, 25 existing skips. Includes Javadoc generation; the CLI option is the JDK 17 workaround documented in the existing POM.

• JMH 1.37, current-version SQL corpus: baseline and patch completed sequentially with three forks, two 10-second warmups and five 1-second measurements per fork. Results are mean ± 99.9% error, in ms/op:

  JDK 17 compiler mode	Baseline	Patch
  Default Graal	60.808 ± 31.867	213.767 ± 368.105
  C2 (-XX:-UseJVMCICompiler)	73.276 ± 37.797	86.913 ± 7.972

  Fork variation and overlapping intervals leave the performance comparison inconclusive. The C2 point estimate is 18.6% slower; these measurements do not establish either an improvement or absence of regression. A steadier environment is needed for a definitive comparison.

Validation used a temporary worktree at upstream commit 4acc89ff, with both changed files byte-identical to this commit, after slow build-artifact hashing in the Documents checkout.

AI assistance: implementation and independent review were performed with AI coding agents. The listed checks were executed locally.

Fixes #2734