Add fluxc compiler skeleton: token definitions and lexer

Introduces the fluxc Rust crate with the first two compiler stages:

- token.rs: define_tokens! macro generates TokenKind enum and its
  Display impl from a single table covering all Flux tokens
  (literals, keywords, operators, punctuation, Eof/Unknown).
  Span (half-open u32 byte range) and Token<'src> (kind + span +
  zero-copy text slice) round out the module.

- lexer.rs: Lexer<'src> produces Token<'src> from a source &str.
  Skips whitespace, // line comments, and /* */ block comments.
  Handles all integer bases (decimal, hex, octal, binary with _
  separators), floats (fractional + exponent), string/char literals
  with escape sequences, and Unicode identifiers via unicode-xid.
  Implements Iterator<Item = Token> and includes 17 unit tests.

Also adds .gitignore (ignores fluxc/target) and expands
examples/fibonacci.flx with an iterative variant.

fluxc/Cargo.lock

@@ -0,0 +1,16 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 4
+
+[[package]]
+name = "fluxc"
+version = "0.1.0"
+dependencies = [
+ "unicode-xid",
+]
+
+[[package]]
+name = "unicode-xid"
+version = "0.2.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ebc1c04c71510c7f702b52b7c350734c9ff1295c464a03335b00bb84fc54f853"