feat: reduce binary size by making D2 support opt-in via build tag

[engineervix]

Problem

The released binary is ~30 MB (stripped). This is large for a CLI documentation tool.

Root Cause

The binary is already built with -ldflags="-s -w" (debug symbols stripped), so there are no easy wins there. The size comes almost entirely from the D2 diagram library and its transitive dependencies:

Package	Role	Symbols
github.com/dop251/goja	Full ECMAScript 5.1 JS VM — bundled by D2	6,597
oss.terrastruct.com/d2	Diagram compiler + SVG renderer	1,689
golang.org/x/image	Image codecs (PNG/JPEG/GIF) — pulled by D2	—
github.com/golang/freetype	Font rendering for D2 text measurement	—
github.com/google/pprof	Profiling — pulled by D2	—

Everything else (cobra, goldmark, chroma, fsnotify, toml) is cheap. Without D2, the binary would be roughly 8–12 MB.

Proposed Fix: Build Tag

Add //go:build d2 to internal/parser/d2.go and a no-op stub for default builds:

// internal/parser/d2_stub.go — compiled by default
//go:build !d2

package parser

import "github.com/yuin/goldmark"

func NewD2Extension() goldmark.Extender { return noopExtension{} }

// internal/parser/d2.go — only compiled with -tags d2
//go:build d2

package parser
// ... existing implementation unchanged

The D2 import in go.mod would become conditional (or stay as-is since unused imports are dead-code eliminated by the linker when the build tag excludes the file).

Default install (~10 MB):

go install github.com/engineervix/kwelea@latest

With D2 diagram support (~30 MB):

go install -tags d2 github.com/engineervix/kwelea@latest

D2 fenced blocks (```d2) in the default build would fall back to rendering as syntax-highlighted plain text.

Other Options Considered

Option	Savings	Notes
Build tag (recommended)	~20 MB	Keeps feature, opt-in
Shell out to d2 CLI	~20 MB	Requires separate d2 install
Drop D2 entirely	~20 MB	Cleanest; d2 blocks become code
UPX compression	~40–50%	Adds startup latency; not portable

Acceptance Criteria

• Default go install produces a binary ≤ 12 MB
• go install -tags d2 produces a full-featured binary (D2 diagrams work)
• just build uses the default (no D2) build
• just build-d2 (new recipe) builds with D2 support
• README documents both install variants
• d2 fenced blocks in default build render gracefully as code