fix(fat-agent): execute bundled probes end to end

[jbachorik]

Summary

• make META-INF/btrace-probes/<binary-name>.class the canonical standalone bundled-probe layout
• resolve both explicit probes= and configurator-selected probes through that layout
• validate Java binary names and fail agent startup for invalid, missing, or unloadable named probes
• make Gradle staging validate nested names, missing inputs, and task ordering before building the fat agent
• update the user guides from the former silent-no-op warning to the working workflow

Fixes #878

Verification

• full :btrace-agent:test suite
• focused runtime tests for nested/inner names, traversal rejection, and fatal missing-resource behavior
• focused Gradle TestKit build checks for nested layout, missing names, and invalid names
• launched-JVM integration fixture observes BUNDLED_PROBE_EXECUTED, application output, BUNDLED_PROBE_TORN_DOWN, and exit code 0
• ./gradlew spotlessCheck
• :btrace-agent:compileJava and :btrace-gradle-plugin:compileGroovy

Baseline limitations

The complete :btrace-gradle-plugin:test run reaches five unrelated failures already present on develop: two tests use the removed registry DSL, one expects the removed registry(...) source, and two extension fixtures define a non-abstract ShadowJar incompatible with Gradle 9.5. The agent and plugin builds also pin JUnit 6.1.2 while compiling tests for Java 8/11, so verification used temporary local JUnit 5.14.4 pins and a Java 17 TestKit launcher; none of those temporary changes is included in this PR.

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