feat: expose type-based styling as a system variable

[apstndb]

Background

PR #557 introduced StyledCell and the typeStyles field on systemVariables, enabling type-based ANSI styling for query result cells. However, typeStyles is currently a private field with no way to configure it — it defaults to nil (no styling).

Goal

Expose type-to-style mapping as a system variable CLI_TYPE_STYLES so users can configure how different Spanner types are displayed in table output.

Design

Format

LS_COLORS/GCC_COLORS style — colon-separated key=value pairs:

SET CLI_TYPE_STYLES = "STRING=green:INT64=bold:FLOAT64=bold:BOOL=yellow:TIMESTAMP=cyan:NULL=dim"

Keys

Spanner type names (uppercase) matching TypeCode enum names, plus NULL:

BOOL, INT64, FLOAT32, FLOAT64, NUMERIC, STRING, BYTES, JSON,
DATE, TIMESTAMP, ARRAY, STRUCT, PROTO, ENUM, INTERVAL, UUID, NULL

• ARRAY, STRUCT are styled by their outer type code (not element type)
• NULL overrides the default dim styling on NullCell. When CLI_TYPE_STYLES is set but NULL is not specified, the current default dim is preserved.

Values

Semicolon-separated SGR parameters, supporting both raw numbers and named colors/attributes:

Named	SGR	Named	SGR
bold	1	black	30
dim	2	red	31
italic	3	green	32
underline	4	yellow	33
blink	5	blue	34
reverse	7	magenta	35
hidden	8	cyan	36
strikethrough	9	white	37

Raw SGR numbers also accepted for 256-color (38;5;N) and truecolor (38;2;R;G;B).

Examples:

CLI_TYPE_STYLES = "STRING=green:INT64=bold:NULL=dim"
CLI_TYPE_STYLES = "STRING=1;32:TIMESTAMP=38;5;214"
CLI_TYPE_STYLES = "STRING=bold;green:BOOL=yellow"

Default

Empty string (no type styling) — preserves current behavior. NullCell retains its hardcoded dim when CLI_TYPE_STYLES is unset or empty.

Interaction with CLI_STYLED_OUTPUT

CLI_TYPE_STYLES only takes effect when CLI_STYLED_OUTPUT resolves to styled mode (Auto+TTY, or True). In non-styled contexts (MCP, piped output, False), RawText() is used regardless of CLI_TYPE_STYLES.

Tasks

• Implement parser for the TYPE=STYLE colon-separated format with named color/attribute support
• Register CLI_TYPE_STYLES as a system variable in system_variables_registry.go
• Wire parsed typeStyles map into systemVariables.typeStyles
• Allow NULL key to override NullCell default dim style
• Add unit tests for parsing (valid, invalid, edge cases)
• Add integration-level tests verifying styled output

References

• StyledCell type: internal/mycli/format/config.go
• typeStyles field: internal/mycli/system_variables.go
• spannerRowToRow (cell type selection): internal/mycli/row_iter.go
• Prior art: LS_COLORS, GCC_COLORS, GREP_COLORS (colon-separated key=value with SGR values)