Resolve post-0.8.0 issue queue and add coverage tooling

[ikraamg]

Summary

Six independent pieces of work from the post-0.8.0 backlog, one commit each, plus
two follow-up commits. Every change ships with specs; the full suite is green and
RuboCop is clean.

Change	Resolves
trmnlp init no longer creates read-only project files	#83
Payload-size badge is colour-coded by size	#67
Non-JSON poll responses are exposed as {{ data }}	#81
trmnlp build --png renders a PNG per view	#92
CLI output is TTY-aware coloured	#33
SimpleCov coverage tracking + lint-check specs	—

Details

Fixed init creating read-only files — #83

init copied templates with a bare FileUtils.cp, which preserves the source file
mode. On read-only gem installs — NixOS stores gems under /nix/store at 0444 —
the generated project was not editable. The copy now adds owner-write while keeping
any executable bit, so bin/trmnlp stays runnable.

Payload-size colour coding — #67

The merge-variable payload size was shown as plain text. The badge now turns yellow
from 75 KB and red from 100 KB, so an oversized payload is obvious before the plugin
misbehaves in production.

Non-JSON polling key — #81

A text/html or text/plain poll response that is not secretly JSON was exposed to
the template as {{ text }}, while the hosted service uses {{ data }}. They now
match, so a plugin previews locally the same way it runs in production.

trmnlp build --png — #92

build only wrote HTML. build --png now screenshots each view through the existing
quantization pipeline and writes a PNG alongside. --width, --height, and
--color-depth flags override the defaults for parity with the serve PNG route. The
headless-Firefox factory moved out of serve into a shared FirefoxDriver module,
since build is its second consumer.

TTY-aware coloured output — #33

Reporter hand-rolled ANSI escape codes and emitted them unconditionally, so colour
leaked into piped output as unreadable sequences. Colour is now suppressed off a
terminal and extended to warnings and the fatal-error message. (A colour gem was
trialed, then dropped in favour of a four-line hand-rolled colorizer to avoid three
transitive dependencies — see the commit history.)

Coverage tracking + lint specs

SimpleCov now runs from spec_helper; CI gates on a 90% line-coverage floor
(currently 92.5%). The 11 lint checks that had no dedicated spec — previously
exercised only incidentally through the lint command — each gained a focused
pass/fail spec.

Closes

Closes #33
Closes #67
Closes #81
Closes #83
Closes #92

Test plan

• bundle exec rspec — 268 examples, 0 failures
• bundle exec rubocop — 111 files, no offenses
• SimpleCov floor enforced in CI — CI=true bundle exec rspec exits 0 at 92.5%
• trmnlp build --png against examples/hn-stories — four 800×480 PNGs, real render
• trmnlp lint piped — no ANSI codes; in a terminal — coloured