log: support multiple outputs with independent formats

[raphael]

Problem

log.WithOutput(...) + log.WithFormat(...) configure one formatter for one writer.

A common production setup needs multiple destinations with independent formats:

• stdout: human-friendly ANSI terminal logs
• file: machine/grep-friendly plain text or JSON

Using io.MultiWriter(os.Stdout, file) tees already-formatted bytes, so ANSI escapes leak into the file.

Solution

Add a first-class multi-sink concept:

• log.Output: pairs a Writer with a Format.
• log.WithOutputs(...): configures one or more outputs; each log entry is formatted and written to every output.

This makes formatting a property of the destination, not of the global logger.

API

New

ctx := log.Context(ctx, log.WithOutputs(
    log.Output{Writer: os.Stdout, Format: log.FormatTerminal},
    log.Output{Writer: logfile,  Format: log.FormatJSON},
))

Backwards compatibility

This PR keeps the existing options:

• log.WithOutput(w) mutates the first output writer.
• log.WithFormat(f) mutates the first output format.

So existing code continues to compile and behave as before.

Examples

ANSI stdout + JSON file

logFile, err := os.OpenFile("app.log", os.O_CREATE|os.O_APPEND|os.O_WRONLY, 0o644)
if err != nil {
    // handle error
}

defaultCtx := log.Context(context.Background(), log.WithOutputs(
    log.Output{Writer: os.Stdout, Format: log.FormatTerminal},
    log.Output{Writer: logFile,  Format: log.FormatJSON},
))

log.Infof(defaultCtx, "hello")

Keep legacy single output configuration

ctx := log.Context(context.Background(),
    log.WithOutput(os.Stderr),
    log.WithFormat(log.FormatText),
)

Implementation notes

• Internals now store outputs as []log.Output.
• Writing fans out across outputs; behavior remains consistent with prior versions (write errors are not surfaced).

Tests

• Updated existing tests/call sites to continue exercising legacy WithOutput/WithFormat through the first-output behavior.
• Added tests for:
  • multi-output fanout (TestMultipleOutputs)
  • WithOutputs validation panics
  • deterministic coverage of the terminal default path in defaultOptions.

Test plan

• go test ./...

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.94%. Comparing base (a647621) to head (88dd73d).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #634      +/-   ##
==========================================
+ Coverage   86.73%   86.94%   +0.21%     
==========================================
  Files          39       39              
  Lines        1990     2007      +17     
==========================================
+ Hits         1726     1745      +19     
+ Misses        239      238       -1     
+ Partials       25       24       -1     

Flag	Coverage Δ
micro	86.94% <100.00%> (+0.21%)	⬆️

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