Fix broken Ponder build dependency and add PlayerRecord/ExperienceCalculator unit tests

[dmccoystephenson]

Summary

This PR does two coupled things — the second needed the first to actually verify in CI:

1. Fixes the build. pom.xml pinned preponderous:ponder:v0.14-alpha-2, which is broken two ways: that tag doesn't exist in Dans-Plugins/Ponder anymore, and the groupId:artifactId casing (preponderous:ponder) was never resolvable via jitpack (Ponder's own pom.xml declares preponderous:Ponder, and jitpack in practice only serves it under com.github.Dans-Plugins:Ponder). This has been breaking mvn compile/CI on main for everyone (confirmed: the last CI run on main failed with the same error, and none of the other 8 open PRs in this repo have any CI checks reported at all). Switches to com.github.Dans-Plugins:Ponder:v0.14 — the nearest existing tag whose package layout matches this project's existing imports exactly, so no source changes were needed. Related to Update Ponder dependency to latest release #116 but does not close it — see the comment I left there on why a true "latest release" upgrade is a separate, larger, breaking migration.
2. Bootstraps this repo's first src/test tests, despite JUnit 4 + Mockito already being wired into pom.xml with nothing using them:
   • PlayerRecordTest: skill-level get/set, experience get/set, getKnownSkills/isKnown, getOverallSkillLevel (filters inactive/removed skills), incrementExperience's no-level-up and max-level-skip branches, checkForLevelUp below-threshold behavior, and a save()/load() round-trip.
   • ExperienceCalculatorTest: the getExperienceRequiredForLevelUp formula (level 0, level 1, zero factor, general scaling, int-truncation).
   • No production code behavior was changed by the test PR itself — this is characterization coverage (Stage B of the dev loop: unit-test expansion).

Known gap, left uncovered on purpose: PlayerRecord.learnSkill(...)/levelUp(...) call Bukkit.getPlayer(...) (a static Bukkit accessor). The project pins mockito-core:3.12.4 with no mockito-inline, so stubbing that static call isn't available without adding a new test dependency — left out of scope here. Auto-learning an unknown skill and the leveling-up branch of checkForLevelUp remain covered only by the manual .testcontainer Spigot smoke harness.

Test plan

• mvn clean package (the exact command CI runs) — green locally after the dependency fix: BUILD SUCCESS, all 18 new tests pass.
• mvn test — Tests run: 18, Failures: 0, Errors: 0, Skipped: 0.
• Verified via a scratch clone of the Ponder repo at both v0.14 and v0.15-alpha-2 that v0.14's package layout matches this project's imports exactly, while v0.15-alpha-2 renames packages (documented in the commit message and issue Update Ponder dependency to latest release #116 comment).

Related: #116 (not closed — see above)

---

This PR description was drafted during a Gardener session (Stephenson-Software/gardener).

[dmccoystephenson]

Self-review rubric:

• Scope: PASS — every changed file (pom.xml, CHANGELOG.md, 2 new test files) is necessary for one of the two things this PR does (unblock the build, add characterization tests); no unrelated formatting/renames/comment churn.
• Tests-new: N/A (PASS) — no new production public methods were added; the PR itself is the new test coverage (18 tests) for existing PlayerRecord/ExperienceCalculator methods.
• Tests-fix (empirical): PASS — for the pom.xml dependency fix, the pre-fix pom.xml was checked out (git checkout a29fb0d~1 -- pom.xml) and mvn compile was run: confirmed FAIL (Could not resolve dependencies ... preponderous:ponder:jar:v0.14-alpha-2). The fixed pom.xml was restored and mvn test was run: confirmed PASS (Tests run: 18, Failures: 0, BUILD SUCCESS). Not scored from reasoning alone.
• Sibling structure: PASS — new test files mirror their production package layout (src/test/java/dansplugins/simpleskills/{playerrecord,experience}/...Test.java), matching this project's existing src/main structure; no sibling test files existed yet to diverge from.
• Sibling renames: N/A — no renames in this PR.
• Docs: PASS — CHANGELOG.md has [Unreleased] entries for both changes. HelpCommand.java, COMMANDS.md, USER_GUIDE.md, CONFIG.md correctly untouched — no command, permission, or config-key behavior changed.
• Issue resolution: PASS — no Closes #N claimed. Update Ponder dependency to latest release #116 is referenced but deliberately not closed (commented on the issue explaining the fix here is a minimal "restore a working build" patch, not the "upgrade to latest release" the issue actually asks for — that's a separate, breaking, larger migration).
• CI: PASS — green on the PR head SHA (gh pr checks 145), confirmed after the dependency fix was pushed.

Repo-specific:

• Skill/stat parity: N/A (no signal) — StatsCommand.java/TopCommand.java/SkillCommand.java not touched by this PR.
• Testcontainer harness untouched implies UNVERIFIED, not PASS: correctly flagged — this PR's PlayerRecordTest explicitly documents (in its class Javadoc and the PR body) that learnSkill/levelUp's live-Bukkit.getPlayer(...) branches are not exercised by these unit tests (mockito-core has no mockito-inline for static mocking) and remain covered only by the manual .testcontainer Spigot smoke harness. Not claimed as tested.

Note for the merge gate: this PR modifies pom.xml, which is on this repo's do-not-auto-merge list (dependency changes affect the shaded JAR/release artifact). It is being left open for human review/merge rather than merged autonomously, even though CI is green and the rubric above passes.

---

This comment was drafted during a Gardener session (Stephenson-Software/gardener).