release: bump to v0.3.0

[viict]

0.2.0 → 0.3.0, and the description of what 0.3.0 actually reads.

The version, in the three places it is written

• Cargo.toml — what tread --version prints
• Cargo.lock — its own entry, updated by cargo build
• README.md — the cargo tree sample under Zero dependencies

The branch originally moved only the first two; the README sample still said
tread v0.2.0. Nothing in the build catches that, which is why
docs/releasing.md (#12) calls that copy out by name.

What it says it reads

Code and plain text landed after the last release, but the product description
did not follow them. All three copies said markdown, CSV and JSON, and the
supported-file-types table listed four formats out of six:

• Cargo.toml's description — this is what crates.io shows
• the --help header in src/cli.rs — this ships inside the binary
• README.md's opening line and the format table, which now carry Code
  (.rs, the four JS/TS spellings, .py .pyi, .java) and Plain text

Both of the first two go out with the tag whether or not they are true, so they
belong in the bump rather than in a follow-up. keywords swapped tui for
code. The --format note now says code is chosen by extension, since it is
not one of the --format values.

Left alone on purpose: the v0.2.0 mentions in CLAUDE.md and
docs/windows.md. Those record when install.ps1 and the interactive reader
were last exercised by hand on real Windows hardware. Bumping them would claim
a verification that has not happened for this release.

Verified

$ ./target/debug/tread --version
tread 0.3.0
$ ./target/debug/tread --help | head -1
tread 0.3.0 — a terminal reader for markdown, CSV, JSON and code

cargo test green (1308 in the lib run plus eight integration binaries, every
one 0 failed) and cargo clippy --all-targets -- -D warnings clean, both with
all three open branches merged together.

Merge order: land #12 first if you can — it is the document describing this
process. Otherwise any order works; the three merge cleanly in every
combination and only README.md is touched by more than one, in disjoint
sections.

Do not tag before ci.yml is green on the merge commit, not just on this
branch — the commit being tagged is a different one once this is merged.