[BUILD] Sync package versions via xtask + scaffold CHANGELOG

[justin13888]

Description

Second PR in the release-automation stack for #18 (stacked on #356). Establishes one repo-wide version with a single patchable source of truth per package, plus a tested tool that writes a new version into all of them at once.

Review after #356 — base is ci/convco-mise.

Version SSoT consolidation

• capsule-media/Cargo.toml — version = "0.1.0" → version.workspace = true, so root Cargo.toml's [workspace.package] version is now the sole SSoT for every Rust crate.
• gradle.properties — new capsule.versionName / capsule.versionCode; capsule-android/build.gradle.kts now reads them (providers.gradleProperty(...)).
• capsule-swift/Project.swift — MARKETING_VERSION (+ CURRENT_PROJECT_VERSION) added to the shared baseSettings; the iOS app had no declared version before.
• Unified the baseline to 0.1.0 everywhere (docs package.json was 0.0.1; Android was 1.0).

xtask crate (new workspace member, publish = false)

• xtask set-version <X.Y.Z> writes the version into every SSoT: root Cargo.toml + pyproject.toml (toml_edit, formatting/comments preserved), both package.json (surgical regex, formatting preserved), gradle.properties (name set, code incremented — it's a monotonic Android build number), and Project.swift (MARKETING_VERSION).
• Each per-format editor is a pure &str -> Result<String> function with unit tests (CLAUDE.md testability). Covered by the existing cargo test --workspace.

Wiring

• justfile — just set-version <X.Y.Z> and just changelog (release group).
• CHANGELOG.md — minimal generated-by-convco stub (the first release populates it; markdownlint-ignored). Note: a full convco changelog here would dump 452 lines of pre-Conventional-Commits history under "Unreleased" since there are no tags yet — the stub is the cleaner baseline.
• ci.yml — xtask/** added to the rust paths-filter.

Verification (local)

• cargo test -p xtask → 7/7 (editors + error paths). cargo clippy -p xtask with CI flags → clean. cargo fmt --check → clean.
• just set-version 0.2.0 then 0.1.0 → every SSoT updated in lockstep; cargo metadata confirms capsule-media/xtask inherit the workspace version; cargo build -p capsule-media builds at 0.1.0.
• Full-workspace just lint-check-rust + just format-check-rust → clean.
• markdownlint → 0 errors (CHANGELOG ignored).
• Kotlin/iOS: Gradle can't run locally (known) and Swift manifest needs Tuist — both validated by CI (kotlin job, build-ios.yml's tuist generate).

Related Issues

Part of #18 (stack 2/3).

Contributor Checklist

• I have signed the Contributor License Agreement.
• My code follows the project's style guidelines according to CONTRIBUTING.md.
• Tests pass (xtask unit tests + workspace clippy/fmt)
• No sensitive info / secrets
• Docs updated if needed

[cloudflare-workers-and-pages]

Deploying capsule with    Cloudflare Pages

Latest commit:	36db49a
Status:	✅  Deploy successful!
Preview URL:	https://37e7cbc8.capsule-22k.pages.dev
Branch Preview URL:	https://build-version-sync-xtask.capsule-22k.pages.dev

View logs