Keep the install ID in a file for programs that are not plugins (0.6.0)

[dmccoystephenson]

Parity with trace-client-python/-js/-csharp/-cpp for programs that are not Spigot plugins: the per-installation ID can now be kept in a file the program names, resolved by the client after its opt-outs.

• Builder.installIdFile(File) — resolved in build() only when the client is enabled, after the environment, server-wide, config and key checks. The first line matching [A-Za-z0-9_.-]{1,255} (whitespace trimmed) is used. A missing file, or one with no such line, gets a new random UUID written to it (parent directories created). A file that exists but cannot be read yields an in-memory ID and is left untouched; a failed write yields an in-memory ID; build() never throws. Fallbacks are logged at FINE.
• Precedence (documented in the Javadoc and README): explicit installId(...) > installIdFile(...) > the server-wide server-id:. When a file is given alongside serverWideConfig(...), the server-wide file still supplies enabled: and tags: but gains no server-id: line.
• TraceClient.installIdFromFile(File) — the same load-or-create step as a static helper, with the README/Javadoc warning the other clients carry: called directly, it writes whatever the opt-outs say.
• 0.6.0: VERSION, file header, pom.xml, README dependency snippet.
• Tests (mirroring the python/js suites): created once with parent dirs and reused; first valid line read and file never rewritten; file with no valid line replaced; unwritable path → in-memory ID, logged, nothing created; unreadable file (a directory) left alone; bad paths (null, blank, NUL) never throw; every opt-out leaves the directory empty; precedence explicit > file > server-wide. mvn -B verify passed locally on JDK 17 (62 tests, release 8).

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https://claude.ai/code/session_014ztkfamsbEqQfcu76me5SL

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drafted by Claude on behalf of Daniel Stephenson