Add logfmt input format

[ChrisJr404]

This adds logfmt as an input format, which is what #207 asked for. logfmt is the space-separated key=value line format that a lot of Go services log in (logrus, the standard library's slog text handler, Heroku, etc.), so being able to point trdsql straight at those logs and query them is handy.

It follows the same shape as the existing LTSV reader: a LogfmtReader that reads the keys from the preread rows to build the columns, so -ir works the same way when later lines introduce new keys. Values can be bare tokens or double-quoted strings (with the usual escapes), and a bare key with no = is read as an empty value. You select it with -ilogfmt or by giving a file a .logfmt extension.

Example:

$ printf 'level=info msg="start request" status=200\n' | trdsql -ilogfmt -oat "SELECT level, status, msg FROM -"
+-------+--------+---------------+
| level | status |      msg      |
+-------+--------+---------------+
| info  |    200 | start request |
+-------+--------+---------------+

I added tests mirroring the LTSV ones (quoted values, bare keys, an unterminated-quote error case, and the file-based preread/read-row cases) plus testdata files, and updated the README. make test passes and go vet/gofmt are clean.

[noborus]

Thank you very much.
Adding logfmt was a TODO item I hadn't gotten around to yet, so I don't see any issue with merging it.

[Copilot]

Pull request overview

Adds logfmt as a new input format to trdsql (requested in #207), enabling direct querying of space-separated key=value log lines commonly produced by Go services.

Changes:

• Introduces a new LOGFMT input format with CLI flag -ilogfmt and .logfmt extension detection.
• Adds LogfmtReader implementation (including quoted value parsing and bare-key handling) and accompanying tests + testdata.
• Updates README documentation to describe logfmt usage and examples.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
trdsql.go	Adds LOGFMT to the Format enum and string conversion.
reader.go	Registers .logfmt extension mapping and NewLogfmtReader factory.
input_logfmt.go	Implements logfmt parsing/reading and preread column discovery.
input_logfmt_test.go	Adds unit tests mirroring LTSV coverage for logfmt parsing and preread/read-row behaviors.
cmd/cmd.go	Adds -ilogfmt flag wiring and format selection updates.
README.md	Documents logfmt input format and updates TOC/section numbering.
testdata/test.logfmt	Adds sample logfmt test data.
testdata/testnull.logfmt	Adds logfmt test data for NULL-replacement scenarios.
testdata/test_quote.logfmt	Adds quoted-value logfmt test data.
testdata/test_indefinite.logfmt	Adds logfmt test data with evolving keys across rows.

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