feat: parameters shared by several graphs, from parameter files

[psyinf]

Why

Several pipelines often need the same tuning values, such as a width or a threshold. Today each .fg graph repeats them inline, so keeping them in sync depends on remembering to edit every copy. This PR moves them into a parameter file that graphs name and share.

What

# alerts.fg
params "tuning.json"
in -> MinLength(minLength=$text.minLength) -> long
long -> Truncate(width=$text.width) -> out

# report.fg
params "tuning.json"   # shared with alerts.fg
params "report.json"   # this graph's own parameters
in -> Truncate(width=$text.width) -> out.text
in -> Classify(classes=$report.classes) -> out.size

DslFilterGraph<std::string, std::string> alerts(dsl::loadGraphProgram("graphs/alerts.fg"));
DslFilterGraph<std::string, std::string> narrow(dsl::loadGraphProgram("graphs/alerts.fg", {{"text", {{"width", 6}}}}));

• $name arguments. Dots walk into nested objects. A parameter can hold any JSON value, so lists and objects can now reach a stage from the DSL, which partly lifts the "flat arguments" limitation.
• params "file.json" lines. Paths are relative to the graph file. Files apply in order, each overriding the ones before it member by member (JSON merge patch). The overrides argument of loadGraphProgram applies last.
• API. New header GraphParameters.hpp, included by DslFilterGraph.hpp:
  • dsl::loadGraphProgram(path, overrides)
  • dsl::parseGraphProgram(text, parameters)
  • dsl::bindParameters(program, parameters)
  • dsl::loadParameters(path)
  • validateDslGraph now also takes a parsed GraphProgram.
• Diagnostics:
  • An unknown name is located and gets a suggestion: 1:22: unknown parameter '$text.widht' — did you mean '$text.width'?
  • A parameter file that is unreadable, invalid JSON or not an object is reported at its params line, with no follow-on "unknown parameter" noise.
  • Unused parameters are deliberately not reported, because a shared file always holds values some graph doesn't use.
• No file reads at parse time. parseGraphProgram(text) only records references and files. Renderings (toAscii / toMermaid / toDot) show unbound parameters as $name. Building a graph from an unbound program reports each parameter as "not set".
• Compatibility. Additive. An edge named params keeps working, because a directive needs a string literal right after the keyword. The deprecated hand-written parser does not support the new syntax (noted in its header and the CHANGELOG).

Docs and sample

• README: new section "Parameters shared by several graphs"; updated feature list and limitations.
• EXAMPLE.md section 11 and new sample apps/tunedPipeline: two .fg files, tuning.json, report.json.
• CHANGELOG and TODO (workaround for nested arguments).

Testing

• tests/FilterGraphTests/ParameterTests.cpp: 16 cases covering binding, nested names, mixing with literals, diagnostics and their locations, syntax errors, rendering, file loading, relative paths, override order and unreadable files.
• Local MSVC debug (ASan): 140/140 tests pass, and all five samples run.

Out of scope (possible follow-ups)

• Reloading parameters into a running graph (hot tuning). Parameters are bound when the graph is built.
• Parameters for the JSON graph format.
• A whole stage config from one parameter, e.g. Stage($preset).