Add Analyze Changed Files action for Git-tracked projects

[Bazooper-blip]

Implements #3: a new Analyze Changed Files action that analyzes every Delphi source file the project's Git repository reports as changed (modified, staged, or untracked), mirroring SonarLint's "Analyze VCS Changed Files".

Changes:

• New DelphiLint.VersionControl unit. TryGetChangedDelphiFiles runs git rev-parse --show-cdup and git status --porcelain --no-renames (headless, output captured over a pipe, 10s timeout) in the analysis base directory, and returns absolute paths of changed Delphi sources that still exist. The porcelain parsing is a separate pure function covered by unit tests (deleted files excluded, malformed lines skipped, quoted paths unescaped).
• The repository root is resolved from the relative --show-cdup rather than --show-toplevel, because the latter canonicalizes mapped/substituted drives to physical paths that don't match the IDE's file paths — which would make the analyzer's base-directory filter silently drop every file. (Found this the hard way on a subst'd drive.)
• New ActionAnalyzeChangedFiles in the plugin, wired into the DelphiLint main menu, the tool window's popup menu, IDE Insight, and toolbar customization/cleanup, with the same enablement rules as the other analyze actions. "Save before analysis" saves only open editors whose files are in the changed set.
• If the project isn't in a Git repository (or git isn't on the PATH), the action shows the same style of message dialog as the existing "no open project" cases.

Design choices to sanity-check in review:

• Git-only, via the PATH git executable, rather than the ToolsAPI version-control services — simpler, and matches what most git-based tooling does; other VCSes could be added behind the same function later.
• Untracked files are included, since they're the classic "about to be committed for the first time" case.
• The action stays enabled even when the project isn't in a repository (checking would mean running git on every action-state refresh); clicking explains why nothing can be analyzed.

Testing: new DelphiLintTest.VersionControl DUnitX fixture for the porcelain parsing (4 tests, all passing; full suite passes at the same rate as master). The git integration itself was exercised end-to-end with a console harness against a real repository — from the repo root, from subdirectories, on a substituted drive, and against a non-repository directory. I have not yet exercised the action inside the IDE itself.