[MSHADE-265] - Add module-info merging support

[wilx]

Summary

Add opt-in merging of Java module descriptors with:

<moduleInfoMode>merge</moduleInfoMode>

The default remains discard, preserving the plugin's existing behavior.

The merge follows a source-amalgamation model: the primary artifact supplies the
module identity, while the shaded contents are treated as if their sources had
been relocated and compiled into that module. By default, the primary module's
exports and opens remain its public boundary. Users can opt into the effective
boundary of embedded modules or add explicit exports, opens, requirements, and
service uses where source-level intent cannot be inferred safely.

The processor:

• removes requirements on modules absorbed into the shaded artifact;
• uses only each release view's configured embedded identity, preserving external
  requirements that collide with inactive descriptor or filename-derived names;
• preserves open-module semantics by emitting no explicit opens for an open primary
  and materializing retained packages from embedded open modules when boundary
  merging is requested;
• infers external reads and service declarations from embedded explicit and
  automatic modules;
• applies configured relocations to packages, classes, providers, and services;
• analyzes automatic-module bytecode and service configuration, failing with
  actionable guidance when dynamic service use cannot be inferred;
• selects automatic-module analysis from the same filtered, release-aware
  descriptor view used for merging;
• resolves platform ownership against ct.sym and jmods from a selected JDK,
  with optional toolchain requirements for release-aware analysis; and
• fails when filtering or minimization would leave an invalid module descriptor.

Multi-release inputs are merged as cumulative release views. The implementation
rebuilds ModulePackages, validates the invariants required of versioned module
descriptors, and raises the modular floor when a provider or public-boundary
change cannot be represented at an earlier release. Earlier releases then retain
an automatic-module bridge with a stable Automatic-Module-Name.

Manifest processing now also accepts reactor output directories. Maven can expose
a reactor dependency as target/classes when separate lifecycle tasks such as
package test are requested. The manifest pre-scan reads a directory manifest
when present, instead of attempting to open the directory as a JAR.

Documentation covers the defaults, public-boundary modes, explicit overrides,
toolchain selection, dynamic service use, and modular-floor behavior.

Fixes #629.

Fixes #581.

Fixes #588.

JIRA: MSHADE-265,
MSHADE-215

Validation

• mvn test: 104 tests run, zero failures; one skipped.
• JDK 8 mvn test: 104 tests run, zero failures; 14 modular-runtime tests skipped.
• mvn -Prun-its verify: 86 integration projects passed, one skipped by its
  JRE-version condition.
• The focused module-info-merge project passed when the built plugin was run
  with Maven 3.6.3, current Maven 3.x, and Maven 4.
• The focused MSHADE-215 project fails with Shade 3.6.2 and passes with the
  branch build under Maven 3.6.3, current Maven 3.x, and Maven 4.
• The original MSHADE-215 reproducer failed under Wine 9 with a Windows Temurin
  21 JVM and Maven 3.9.16; the branch build completed successfully in the same
  environment.
• mvn site passed.
• The generated modular JAR passed jar --validate, jar --describe-module,
  and jdeps --check.

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