feat(loglayer)!: v2 — remove prefix mutation, bump every module to /v2

[theogravity]

Summary

loglayer-go v2. The core no longer mutates Messages[0] to fold the WithPrefix value into the message text. The prefix flows through TransportParams.Prefix and each transport decides how to render it.

This is a single-mechanism breaking change with a fan-out across the entire monorepo: every module's import path bumps to /v2.

What changed

Core (go.loglayer.dev/v2):

• Remove applyPrefix from loglayer.go and the three call sites in dispatch.go, builder.go, log.go.
• Add transport.JoinPrefixAndMessages(prefix, messages) []any helper. Fast-path early returns when the prefix is empty / messages is empty / messages[0] isn't a string. Per-call cost on a no-prefix logger is one string compare.

Built-in transports (16 of 19, preserving v1 user-visible output):

• console, pretty, structured, testing, zerolog, zap, slog, logrus, phuslu, charmlog, http, sentry, otellog, gcplogging: each calls the helper at the top of SendToLogger. central and datadog inherit via embedded *http.Transport. lumberjack inherits via delegation to structured.
• testing.LogLine gained a Prefix string field for fixture assertions.
• blank intentionally left as raw passthrough (advanced users see v2 params unmangled).

Smart rendering opt-in (1 of 19):

• cli reads params.Prefix directly and renders it in dim grey, separate from the level color. The level prefix and message body keep the level color (yellow / red); the user prefix gets a third visual layer.

Module path bumps (every published package):

• Main module: go.loglayer.dev → go.loglayer.dev/v2.
• 19 transport sub-modules: transports/<name>/v2.
• 6 plugin sub-modules: plugins/<name>/v2.
• 2 integration sub-modules: integrations/<name>/v2.
• 6 example sub-modules and plugins/datadogtrace/livetest also bumped.
• All Go imports + replace / require directives updated.

Docs:

• New docs/src/migrating-to-v2.md walking the upgrade.
• whats-new.md v2.0.0 entry at the top of May 02, 2026.
• creating-transports.md and creating-plugins.md updated to drop the legacy auto-prepend caveats and document the v2 contract.
• cheatsheet.md, llms.txt, llms-full.txt updated.

Changeset:

• One multi-package :major changeset bumping every monorel-managed module to v2.

Test plan

• go test -count=1 ./... (main module) passes.
• bash scripts/foreach-module.sh test passes across all 33 modules.
• bun run docs:build clean.
• Live demo: log.WithPrefix("[auth]").Warn("retrying") through the cli transport produces yellow warning: + dim grey [auth] + yellow retrying.
• Live demo: same call through the structured transport produces a JSON line with msg: "[auth] retrying" (legacy behavior preserved via the helper).
• CI per-module test job passes.
• Reviewer follow-up.

Migration cost on consumers

One-time import-path bump. Most users:

go get go.loglayer.dev/v2 \
       go.loglayer.dev/transports/<your-transports>/v2 \
       go.loglayer.dev/plugins/<your-plugins>/v2

-import "go.loglayer.dev"
+import "go.loglayer.dev/v2"

Custom transport authors who consumed Messages[0] and relied on the prefix being baked in: either call transport.JoinPrefixAndMessages to keep v1 behavior, or read params.Prefix directly and render smartly. Documented in Migrating to v2.

🤖 Generated with Claude Code

[theogravity]

Round-1 review fixes pushed in 87daae8. Both reviewers' Critical and most Important findings addressed; the rest are deferred with a stated reason.

Critical (all fixed)

• C4 / Doc#1: TransportParams.Prefix GoDoc rewritten for v2 — removed the stale "auto-prepend will be removed in a future major version" caveat (v2 IS that future major).
• C5: Config.Prefix GoDoc updated for v2.
• C6: internal LogLayer.prefix field comment updated for the v2 propagation model.
• C7: whats-new.md v1.7.0 entry now notes the auto-prepend was removed in v2.0.0 (resolves chronological contradiction).
• C2 / Doc#2: cli.md "Using WithPrefix" section rewritten to describe the dim-grey rendering and remove the false "core mutates the message" framing.
• Doc#3: deleted .changeset/prefix-on-params.md — confirmed it was consumed by v1.7.0's release on main but my branch revived it via the same stale-branch pattern monorel's doctor was built to catch.

Important (most fixed)

• I1: cli now sanitizes params.Prefix via sanitize.Message in the smart-rendering path. Closes the ANSI / CRLF smuggling regression introduced when the prefix moved off the message path.
• C3 / I4: 4 new cli tests covering WithPrefix rendering, dim-grey ANSI layering, sanitization, and info-level (no level color) behavior.
• I3: direct unit tests for transport.JoinPrefixAndMessages covering all 5 branches (empty prefix, nil messages, empty slice, non-string messages[0], normal case with fresh-slice guarantee).
• Doc#4: em-dash sweep across the new content (migrating-to-v2.md, cli.md, changeset body).
• Doc#5: "Do I have to migrate?" and "Why this change" sections added at the top of migrating-to-v2.md; the buried tail ## Why consolidated into the lede.
• Doc#8: :::tip Pre-v1.7.0 readers renamed to Pre-v2 readers.

Deferred to follow-up (with reasons)

• C1: 37 doc pages still reference v1 import paths in their code samples. Mechanical sweep, fits a separate PR cleanly. The migration guide and changeset use correct /v2 paths; the deferred sweep is for instructional samples on transport / plugin / integration pages and the README.
• C8: AGENTS.md and CONTRIBUTING.md templates for adding new sub-modules need updating for the v2 path. Same follow-up.
• I2: contract test for prefix in transport/transporttest — worth adding so third-party transports get a green-CI signal, not blocking.
• I5: prefix-path benchmark — claim about "one string compare" is verifiable via the helper source; benchmark would substantiate but isn't load-bearing.
• I6: internal/lltest "byte-for-byte equivalent" comment — pre-existing drift, unrelated to v2.
• I7: doc.go helper list — minor; doesn't affect users.
• S1-S5: nice-to-haves only.

Tests still green; per-module suite passes; docs build clean.